Definition of baby:
a very young child (birth to 1 year) who has not yet begun to walk or talk; "the baby began to cry again"; "she held the baby in her arms"; "it sounds simple, but when you have your own baby it is all so different"
the youngest member of a group (not necessarily young); "the baby of the family"; "the baby of the Supreme Court"
an immature childish person; "he remained a child in practical matters as long as he lived"; "stop being a baby!"
an unborn child; a human fetus; "I felt healthy and very feminine carrying the baby"; "it was great to feel my baby moving about inside"
(slang) sometimes used as a term of address for attractive young women
a very young mammal; "baby rabbits"
a project of personal concern to someone; "this project is his baby"
treat with excessive indulgence; "grandparents often pamper the children"; "Let's not mollycoddle our students!"