An infant is the more formal or synonym for "baby", the very young offspring of a human.
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 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.