estrogen UK /ˈiːstɹəd͡ʒən/US /ˈɛstɹəd͡ʒən/ noun1 •Any of a group of steroids that are secreted by the ovaries and function as female sex hormones.Amies Oelschlager said in a statement that oral contraceptives that contain estrogen may cause side effects such as headaches, nausea, and breast tenderness. — 2023 formsestrogens