Infant is a slightly more formal word for baby, the youngest category of child, meaning a human from birth to age 1.5 years. The word "infant" derives from the Latin word in-fans, meaning "unable to speak." "Infant" is also a legal term meaning minor; that is, a child in general.

A human infant less than 28 days old is a newborn, and a newborn infant is called a neonate during the first three months of life. The term "newborn" includes premature infants and postmature infants, as well as full term newborns.

The newborn's appearance Newborn infant moments after the umbilical cord had been cut.A newborn’s shoulders and hips are narrow, the abdomen protrudes slightly, and the arms and legs are relatively short. The average weight of a full-term newborn is approximately 7 ½ pounds (3.2kg), but can be anywhere from 6-10 pounds (2.7-4.6kg). The average total body length is 14-20 inches (35.6-50.8cm), although premature newborns may be much smaller. The Apgar score is a measure of a newborn's transition from the womb during the first ten minutes of life.

A newborn’s head is very large in proportion to the rest of the body, and the cranium is enormous relative to his or her face. While the adult human skull is about 1/8 of the total body length, the newborn’s is twice that. At birth, many regions of the newborn’s skull have not yet been converted to bone. These “soft spots” are known as fontanels; and the two largest are the diamond-shaped anterior fontanel, located at the top front portion of the head, and the smaller triangular-shaped posterior fontanel, which lies at the back of the head.