Baby Health Glossary

Plain-language definitions of medical and developmental terms parents encounter. Each definition links to related guides and tools on this site.

Last reviewed: July 2026

A

Adjusted age: The age of a premature baby calculated from the original due date rather than the actual birth date. Used for developmental milestones until age 2.; Adjusted Age Calculator
Anaphylaxis: A severe, potentially life-threatening allergic reaction that occurs within seconds or minutes of exposure to an allergen. Requires immediate emergency treatment with epinephrine.
Anterior fontanelle: The soft spot on the top of a baby's head where the skull bones have not yet fused. Typically closes between 12 and 18 months of age.
Apgar score: A quick assessment performed at 1 and 5 minutes after birth that evaluates a newborn's heart rate, breathing, muscle tone, reflexes, and skin color on a scale of 0 to 10.
Apnea: A pause in breathing lasting more than 20 seconds, or a shorter pause accompanied by a drop in heart rate or change in skin color. More common in premature infants.
ASQ (Ages and Stages Questionnaire): A standardized developmental screening tool completed by parents that assesses communication, gross motor, fine motor, problem-solving, and personal-social skills at specific ages.; Developmental Assessment Guide
Atopic dermatitis: A chronic inflammatory skin condition commonly known as eczema. Causes dry, itchy, red patches of skin. Common in infants and often appears on the cheeks, arms, and legs.; Skin Guide

B

Babinski reflex: A normal newborn reflex where stroking the sole of the foot causes the toes to fan out and the big toe to extend upward. This reflex disappears by 12 to 24 months as the nervous system matures.
Bilirubin: A yellow pigment produced when red blood cells break down. High levels in newborns cause jaundice (yellowing of the skin and eyes). Measured with a blood test or transcutaneous bilirubinometer.; Jaundice Triage
Bonding: The emotional attachment that develops between a parent and baby. Strengthened through skin-to-skin contact, feeding, eye contact, and responsive caregiving. Bonding is a process, not an instant event.; Mental Health
Bronchiolitis: A common respiratory infection in infants caused by viruses (most often RSV) that causes swelling and mucus buildup in the small airways of the lungs. Most common in babies under 12 months.; Breathing Triage

C

Cephalohematoma: A collection of blood between the skull bone and its covering (periosteum) caused by pressure during delivery. Appears as a soft bump on the head and resolves on its own within weeks to months.
Circumcision: Surgical removal of the foreskin covering the tip of the penis. An elective procedure typically performed in the first few days of life. The AAP states benefits outweigh risks but does not universally recommend it.
Cluster feeding: A pattern where a baby feeds several times in quick succession, often in the evening. Normal behavior that helps increase milk supply and is especially common during growth spurts.
Colic: Excessive crying in an otherwise healthy infant, typically defined as crying for more than 3 hours a day, more than 3 days a week, for more than 3 weeks. Usually peaks around 6 weeks and resolves by 3 to 4 months.; Crying Triage
Colostrum: The first form of breast milk produced during pregnancy and the first few days after birth. Thick and yellowish, rich in antibodies and nutrients. Sometimes called "liquid gold."; Feeding Guide
Congenital: Present at birth. A congenital condition may be inherited (genetic) or caused by factors during pregnancy. Examples include congenital heart defects and congenital hip dysplasia.
Cradle cap: Yellowish, crusty, or greasy patches on a baby's scalp. A form of seborrheic dermatitis that is harmless and usually clears up on its own within a few months.; Skin Guide
Cross-reactivity: When the immune system reacts to proteins that are similar across different foods. For example, a baby allergic to cow's milk may also react to goat's milk because the proteins are structurally similar.; Food Allergens
Croup: A viral infection that causes swelling of the upper airway, producing a distinctive barking cough and sometimes a high-pitched sound when breathing in (stridor). Most common in children 6 months to 3 years.; Breathing Triage

D

Developmental milestone: A skill or behavior that most children can do by a certain age. Categories include gross motor, fine motor, language, cognitive, and social-emotional. Milestones are ranges, not deadlines.; Milestones
Developmental regression: The loss of skills a child previously had. While brief regressions during illness or stress are common, persistent loss of skills warrants evaluation.; Regression Guide
Diastasis recti: A separation of the left and right sides of the abdominal muscles (rectus abdominis) that occurs during pregnancy. Common postpartum and may improve with targeted exercises.
DTaP: A combination vaccine that protects against diphtheria, tetanus, and pertussis (whooping cough). Given in 5 doses at 2, 4, 6, and 15-18 months, and 4-6 years.; Vaccine Tracker

E

Eczema: A group of conditions that cause inflamed, itchy, and often red skin. Atopic dermatitis is the most common type in babies. Triggered by irritants, allergens, dry air, or stress.; Skin Guide
Engorgement: Swelling and hardening of the breasts caused by an increase in blood flow and milk production. Common in the first week postpartum and during weaning. Treated with frequent feeding or expressing.; Feeding Guide
Expressive language: The ability to communicate thoughts and needs using words, gestures, or signs. Distinct from receptive language (understanding). Delays in expressive language are one of the most common parental concerns.

F

Failure to thrive: A term used when a baby is not gaining weight or growing at the expected rate. May be caused by feeding difficulties, medical conditions, or inadequate caloric intake. Requires medical evaluation.; Growth Calculator
Febrile seizure: A convulsion triggered by a rapid rise in body temperature, most commonly during a fever. Occurs in 2-5% of children ages 6 months to 5 years. Usually lasts less than 5 minutes and does not cause brain damage.; Febrile Seizures Guide Fever Triage
Fontanelle: The soft spots on a baby's skull where the bones have not yet fused together. The anterior (front) fontanelle is the largest and typically closes between 12 and 18 months. The posterior (back) fontanelle closes by 2 to 3 months.
Foremilk and hindmilk: Foremilk is the thinner milk at the beginning of a feed. Hindmilk is the fattier milk that comes later. Both are important. An imbalance (too much foremilk) can cause green, frothy stools.; Poop Guide
FPIES: Food Protein-Induced Enterocolitis Syndrome. A non-IgE food allergy that causes severe vomiting and sometimes diarrhea 2-6 hours after eating a trigger food. Most common triggers are milk, soy, rice, and oats.; Food Allergens

G

Gastroesophageal reflux (GER): The backward flow of stomach contents into the esophagus. Common in infants due to an immature lower esophageal sphincter. Usually resolves by 12-18 months. Different from GERD, which causes complications.
GERD: Gastroesophageal Reflux Disease. When reflux causes troublesome symptoms or complications such as poor weight gain, feeding refusal, or esophageal irritation. Requires medical evaluation and may need treatment.
Gestational age: The age of a pregnancy measured from the first day of the last menstrual period. A full-term pregnancy is 39 to 40 weeks. Premature birth is before 37 weeks.
Gross motor skills: Large-muscle movements such as rolling, sitting, crawling, standing, and walking. Develop in a head-to-toe, center-to-periphery sequence.; Milestones

H

Hand-foot-and-mouth disease: A common viral illness in young children caused by coxsackievirus. Causes sores in the mouth and a rash on the hands, feet, and sometimes buttocks. Highly contagious but usually mild.; Rash Triage
Hemangioma: A benign (non-cancerous) growth of blood vessels that appears as a red or purple raised area on the skin. Most common birthmark. Grows during the first year, then gradually shrinks and fades over several years.; Skin Guide
Hib: Haemophilus influenzae type b. A bacteria that can cause meningitis, pneumonia, and other serious infections in young children. Prevented by the Hib vaccine given at 2, 4, 6, and 12-15 months.; Vaccine Tracker
Hirschsprung disease: A condition present at birth where nerve cells are missing from the end of the large intestine, causing severe constipation or bowel obstruction. Requires surgical treatment.
Hypertonia: Abnormally increased muscle tone or stiffness. The baby may feel rigid, have difficulty bending limbs, or arch backward frequently. May indicate a neurological condition and warrants evaluation.
Hypotonia: Abnormally decreased muscle tone, sometimes called "floppy baby." The baby may feel limp when held and have difficulty holding up the head. Has many possible causes and warrants evaluation.

I

Intussusception: A serious condition where one part of the intestine telescopes into an adjacent part, causing a blockage. Symptoms include sudden severe abdominal pain, vomiting, and currant-jelly stools. Requires emergency treatment.

J

Jaundice: Yellowing of the skin and whites of the eyes caused by elevated bilirubin levels. Very common in newborns (affects about 60% of full-term babies). Usually harmless but severe cases need phototherapy treatment.; Jaundice Triage

K

Kangaroo care: Skin-to-skin contact between a parent and baby, with the baby placed on the parent's bare chest. Regulates body temperature, heart rate, and breathing. Especially beneficial for premature infants.

L

Lactation consultant: A healthcare professional specializing in breastfeeding support. An IBCLC (International Board Certified Lactation Consultant) is the gold-standard credential. Can help with latch, supply, and other feeding challenges.; Feeding Guide
Lanugo: Fine, soft body hair that covers the fetus during pregnancy and may still be present on a newborn, especially premature babies. Falls out naturally within the first few weeks to months.
Latch: How a baby attaches to the breast for feeding. A good latch means the baby has a wide mouth around the areola (not just the nipple), with the chin touching the breast and the lips flanged outward.; Feeding Guide
Let-down reflex: The release of milk from the breast triggered by the hormone oxytocin. Stimulated by nursing, pumping, or even thinking about the baby. May cause a tingling sensation and milk leaking from the opposite breast.

M

M-CHAT (Modified Checklist for Autism in Toddlers): A validated screening tool for autism spectrum disorder used at the 18-month and 24-month well-child visits. Completed by parents. A positive screen indicates further evaluation is needed, not a diagnosis.; Developmental Assessment Guide
Mastitis: An infection of the breast tissue that causes pain, swelling, warmth, and redness. Often occurs when a milk duct becomes blocked. May cause flu-like symptoms. Usually treated with antibiotics and continued breastfeeding.; Feeding Guide
Meconium: The first stool passed by a newborn, typically within 24-48 hours of birth. Dark green to black in color, thick and sticky. Composed of materials ingested in utero. Transition to normal stools occurs within a few days.; Poop Guide
Milia: Tiny white bumps on a newborn's nose, chin, or cheeks caused by trapped skin cells. Very common and harmless. Resolve on their own within a few weeks without treatment.; Skin Guide
Moro reflex: Also called the startle reflex. When a newborn feels a sudden movement or loud noise, they throw their arms out, spread their fingers, and then bring their arms back in. Normal until about 4 months of age.

N

Neonatal: Relating to the first 28 days of life. The neonatal period is a critical time for adaptation to life outside the womb.
NICU: Neonatal Intensive Care Unit. A specialized hospital unit for newborns who need intensive medical care, including premature babies and those with medical conditions.; NICU Discharge Checklist

O

Object permanence: The understanding that objects continue to exist even when they cannot be seen. Develops around 4 to 8 months of age. Tested by hiding a toy under a blanket. Related to separation anxiety.

P

Palmar grasp: A reflexive grasping motion where a baby wraps all four fingers around an object placed in the palm. Present at birth. Disappears around 5 to 6 months as voluntary grasping develops.; Milestones
Percentile: A measure indicating where a child falls compared to other children of the same age and sex. The 50th percentile is average. Being at the 10th or 90th percentile is not inherently concerning — tracking the trend over time matters more.; Growth Calculator
Phototherapy: Treatment for jaundice using special lights that help break down bilirubin in the baby's skin. The baby is placed under the light wearing only a diaper with eye protection.
Pincer grasp: The ability to pick up small objects using the thumb and index finger. Develops between 8 and 12 months. Important for self-feeding and fine motor development.; Milestones
Plagiocephaly: A flat spot on one side of a baby's head caused by sustained pressure. Often caused by sleeping in one position. May be treated with repositioning, tummy time, or in some cases a corrective helmet.; Plagiocephaly Guide
Posterior fontanelle: The smaller soft spot at the back of a baby's head. Typically closes by 2 to 3 months of age.
Postpartum depression: A mood disorder that affects some mothers after giving birth. More severe and longer-lasting than baby blues. Symptoms include persistent sadness, anxiety, difficulty bonding, and changes in sleep or appetite. Treatable with therapy and medication.; Mental Health Check-in
Preeclampsia: A pregnancy complication characterized by high blood pressure and signs of damage to organ systems, most often the liver and kidneys. Usually begins after 20 weeks of pregnancy. Can be life-threatening if untreated.
Primitive reflexes: Automatic movement patterns present at birth that are controlled by the brainstem. Include rooting, sucking, Moro, grasping, and stepping reflexes. Gradually disappear as the brain matures during the first year.
Pyloric stenosis: A condition where the muscle between the stomach and small intestine thickens, blocking food from passing through. Causes projectile vomiting, usually starting between 2 and 8 weeks of age. Treated with surgery.

R

Receptive language: The ability to understand language, including following directions and comprehending questions. Develops before expressive language. A child typically understands many more words than they can say.
Reflux: The backward flow of stomach contents into the esophagus, causing spit-up or vomiting. Very common in babies due to an immature digestive system. Usually peaks at 4 months and resolves by 12-18 months.; Vomiting Triage
Rooting reflex: A newborn reflex where the baby turns toward anything that touches their cheek or mouth and opens their mouth in search of a nipple. Helps with breastfeeding. Disappears around 4 months.
RSV: Respiratory Syncytial Virus. A common respiratory virus that causes cold-like symptoms in most children but can cause bronchiolitis or pneumonia in infants, especially premature babies. Peak season is fall and winter.; Breathing Triage

S

Separation anxiety: Distress when a child is separated from a primary caregiver. A normal developmental stage that typically begins around 6-8 months, peaks between 10-18 months, and gradually decreases.
SIDS: Sudden Infant Death Syndrome. The unexplained death of a seemingly healthy baby, usually during sleep. Risk is reduced by placing baby on their back to sleep, using a firm flat surface, and keeping the sleep area clear of soft objects.
Sleep regression: A period when a baby who was sleeping well suddenly has difficulty falling asleep, wakes more at night, or resists naps. Common at 4, 8-10, 12, 18, and 24 months. Usually resolves within 2-6 weeks.; Sleep Guide
Stranger anxiety: Fear or wariness of unfamiliar people that typically develops around 6-8 months of age. A normal sign of cognitive development indicating the baby can distinguish familiar from unfamiliar faces.
Stridor: A high-pitched, noisy breathing sound caused by a partially blocked airway. Can be heard during inhalation. Common causes include croup and laryngomalacia. May require medical evaluation.; Breathing Triage

T

Thrush: A common yeast infection (Candida) in babies that appears as white patches on the tongue, gums, or inner cheeks. Can also cause diaper rash. Treated with antifungal medication.
Tongue-tie: A condition (ankyloglossia) where the band of tissue connecting the tongue to the floor of the mouth is shorter than usual, potentially restricting tongue movement. Can affect breastfeeding. May be treated with frenotomy.
Torticollis: A condition where a baby's neck muscles are tight on one side, causing the head to tilt or turn to one side. Often present at birth or develops in the first few months. Treated with stretching exercises and tummy time.
Tummy time: Supervised time spent on the stomach while awake. Essential for developing head, neck, and upper body strength. Recommended starting from birth for short periods, building up as the baby grows.; Tummy Time Guide

U

Umbilical cord stump: The remaining portion of the umbilical cord attached to the newborn's belly button after birth. Dries out and falls off naturally within 1 to 3 weeks. Keep clean and dry; do not pull it off.

V

Vernix: A white, waxy coating on a newborn's skin at birth. Protects the skin in the womb. The WHO recommends leaving it on for at least 6 hours as it has antimicrobial properties and moisturizes the skin.

W

Well-child visit: Routine preventive care appointments with a pediatrician at regular intervals during the first 3 years. Include physical exams, growth measurements, developmental screening, and vaccinations.; Doctor Visits
Wonder weeks: A theory describing predictable periods of developmental leaps in the first 20 months of life. During these periods, babies may be fussier than usual as their brain undergoes rapid changes.; Wonder Weeks Calculator

Bottom line

Medical terminology can feel overwhelming, especially for new parents. This glossary provides plain-language definitions for the terms you're most likely to encounter at pediatrician visits, in parenting resources, and on this site. If your doctor uses a term you don't understand, ask them to explain — they want you to feel informed.

Trust your instincts. If something feels wrong, reach out to your pediatrician. Worrying about your baby means you care — that is a good thing.

All content follows our editorial policy and is reviewed against published clinical guidelines.

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