Birth is the act or process of bearing or bringing forth offspring is also referred to in technical contexts as parturition. In mammals, the process is initiated by hormones which cause the muscular walls of the uterus to contract expelling the fetus at a developmental stage when it is ready to feed and breathe. In some species the offspring is precocial and can move around almost immediately after birth but in others it is altricial and completely dependent on parenting. Define birth. birth synonyms, birth pronunciation, birth translation, English dictionary definition of birth. beginning of existence; origin: the birth of the blues Not to be confused with: berth – a built-in bed or bunk: Joey wants to sleep on the top.