An infant (from the Latin word infans, meaning "unable to speak" or "speechless") is the more formal or specialised synonym for "baby", the very young offspring of a human. The term may also be used to refer to juveniles of other organisms.

A newborn is, in colloquial use, an infant who is only hours, days, or up to one month 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, full term, and postmature infants; before birth, the term "fetus" is used. The term "infant" is typically applied to young children under one year of age; however, definitions may vary and may include children up to two years of age. When a human child learns to walk, the term "toddler" may be used instead.In first world nations, the average total body length of a newborn is 35.6–50.8 cm (14.0–20.0 in), although premature newborns may be much smaller.The way to measure a baby's length is to lay the baby down and stretch a measuring tape from the top of the head to the bottom of the heel.