In basic English usage, an infant is defined as a human child at the youngest stage of life, especially before they can walk or simply a child before the age of one.

The term "infant" derives from the Latin word in-fans, meaning "unable to speak." There is no exact definition for infancy. Quite often babies are called infants until they reach the age of one. However, babies are traditionally called "toddlers" when they start to walk whether or not they have reached their first birthday. Daycares with an "infant room" providing care will call all their children in the infant room "infants" even if they are older than a year and/or walking; they will sometimes use the term "walking infant".

"Infant" is also a legal term with the meaning of minor; that is, any child under the age of legal adulthood. A human infant less than a month old is a newborn infant or a neonate. The term "newborn" includes premature infants and postmature infants, as well as full term newborns.