1.NounA natural estrogenic hormone that is a phenolic alcohol estra-1,3,5-triene-3,17-diol (.mw-parser-output .template-chem2-su{display:inline-block;font-size:80%;line-height:1;vertical-align:-0.35em}.mw-parser-output .template-chem2-su>span{display:block;text-align:left}.mw-parser-output sub.template-chem2-sub{font-size:80%;vertical-align:-0.35em}.mw-parser-output sup.template-chem2-sup{font-size:80%;vertical-align:0.65em}C18H24O2) secreted chiefly by the ovaries of all vertebrates, that is a major and the most potent of the naturally occurring estrogens; the semisynthetic esterified compound is used medicinally to treat estrogen deficiency and breast cancer.