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Breast milk is milk produced by human mammary glands during lactation. Its nutrients and other constituents support the infant while its composition changes with physiological context. It is a biological secretion rather than one invariant chemical mixture.

A mammary gland is a glandular system that produces milk and carries it through ducts toward the nipple. Secretory epithelial units make the milk, while surrounding contractile cells help expel it into the duct system. The glandular tissue is part of the breast, alongside fat and supporting connective tissue.

Milk production and milk ejection are different processes. Prolactin promotes milk production, whereas oxytocin promotes contraction associated with milk movement or let-down.^[ref] The distinction gives a useful physiological example: material can already have been synthesized yet still require a separate transport mechanism to reach its outlet. Gland size, secretion rate and release of stored secretion are therefore different observations.