Female sex hormone.
There is no one hormone named "estrogen"; estrogens are hormones that induce or accentuate female sexual characteristics (as well as performing other functions, depending on the specific hormone). Estrogens include estrone, progesterone, and estradiol. "estrogenic" means "tending to induce the same effects that estrogen does"; while chemicals that mimic testosterone are almost nonexistent in nature, our food supply, and our environment, estrogenic chemicals are common. Some of these, such as genestein (found in soy) have gotten considerable press coverage because they are weak estrogens, meaning that while they can occupy an estrogen receptor , they do not stimulate the receptor to as great an extent as the body's own estrogens would, which can reduce one's risk of estrogen-related cancers and tumors, most notably breast cancer. However, for this last effect to occur, the weak estrogens must be present in the body in sufficient quantity so that they displace existing natural estrogens that would otherwise stimulate available estrogen receptors--and the total degree of stimulation produced by the greater number of occupied receptors must be less than would have been produced by the naturally present estrogens alone. Other estrogenic chemicals are found in plastics and pesticides and exert harmful developmental effects by disrupting the normal hormonal events that take place in humans and animals.