Antiestrogen: A substance that can prevent the full expression of estrogen.

Antiestrogens act by exerting antagonistic effects on target tissues (androgens and progestogens act in this way) or by competing with estrogens for access to receptor sites located on the cell surface.

For example, the drug tamoxifen (brand name: Nolvadex) is an antiestrogen that is used in the treatment of breast cancer and to reduce the breast cancer incidence in high-risk women.