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 smallerThe 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.

A newborn's head is very large in proportion to the body, and the cranium is enormous relative to his or her face. While the adult human skull is about one seventh of the total body length, the newborn's is about ¼. Normal head circumference for a full-term infant is 33–36 cm at birth.[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.Newborn on yellow blanket being attended to by a nurseA newborn baby in Indonesia, with umbilical cord ready to be clamped/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.