In basic English usage, an infant is defined as a human child at the youngest stage of life, specifically before they can walk and generally before the age of one^[1] (see also child and adolescent).The term "infant" derives from the Latin word in-fans, meaning "unable to speak." There is no exact definition for infancy. "Infant" is also a legal term with the meaning of minor;^[2] 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.^[3] The term "newborn" includes premature infants, postmature infants and full term newborns.Upon reaching the age of one or beginning to walk, infants are subsequently referred to as "toddlers" (generally 12-36 months). Daycares with an "infant room" often call all children in it "infants" even if they are older than a year and/or walking; they sometimes use the term

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 about 1/4. At birth, many regions of the newborn's skull have not yet been converted to bone, leaving "soft spots" known as fontanels. 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. Later in the child's life, these bones will fuse together in a natural process. A protein called noggin is responsible for the delay in an infant's skull fusion.^[4]

During labour and birth, the infant's skull changes shape to fit through the birth canal, sometimes causing the child to be born with a misshapen or elongated head. It will usually return to normal on its own within a few days or weeks. Special exercises sometimes advised by physicians may assist the process.

Some newborns have a fine, downy body hair called lanugo. It may be particularly noticeable on the back, shoulders, forehead, ears and face of premature infants. Lanugo disappears within a few weeks. Likewise, not all infants are born with lush heads of hair. Some may be nearly bald while others may have very fine, almost invisible hair. Some babies are even born with a full head of hair. Amongst fair-skinned parents, this fine hair may be blond, even if the parents are not. The scalp may also be temporarily bruised or swollen, especially in hairless newborns, and the area around the eyes may be puffy.