An infant (from the Latin word infans, meaning "unable to speak" or "speechless") is the very young offspring of a human or other mammal. When applied to humans, the term is usually considered synonymous with baby, but the latter is commonly applied to the young of any animal. When a human child learns to walk, the term toddler may be used instead.

The term infant is typically applied to young children between the ages of 1 month and 12 months; however, definitions vary between birth and 3 years of age. A newborn is an infant who is only hours, days, or up to a few weeks old. In medical contexts, newborn or neonate (from Latin, neonatus, newborn) refers to an infant in the first 28 days after birth; the term applies to premature infants, postmature infants, and full term infants. Before birth, the term fetus is used.

Infant is also a legal term referring to any child under the age of legal adulthood.

A newborn's shoulders and hips are wide, the abdomen protrudes slightly, and the arms and legs are relatively long with respect to the rest of their body. In first world nations, the average birth weight of a full-term newborn is approximately 3.2 kg.(7 ½ lbs), and is typically in the range of 2.7–4.6 kg (5.5–10 pounds). The average total body length is 35.6–50.8 cm (14–20 inches), although premature newborns may be much smaller. The Apgar score is a measure of a newborn's transition from the uterus during the first minutes after birth.