1. A short polypeptide hormone, C₄₃H₆₆N₁₂O₁₂S₂, released from the posterior lobe of the pituitary gland, that stimulates the contraction of smooth muscle of the uterus during labor and facilitates ejection of milk from the breast during nursing.

2. A synthetic form of this hormone, used as a drug to induce labor and to control postpartum hemorrhage.