Oxytocin: A hormone made in the brain that plays a role in childbirth and lactation by causing muscles to contract in the uterus (womb) and the mammary glands in the breast. Animal studies have shown that oxytocin also has a role in pair bonding, mate-guarding, and social memory.

Oxytocin is a peptide (a compound consisting of 2 or more amino acids) that is secreted by the hypothalamus and transported to the posterior lobe of the pituitary gland at the base of the brain. Oxytocin is related to another hormone called vasopressin.