This is the usual American spelling of the same hormone British English writes as oestrogen — not a second chemical, only a second spelling (American English dropped the o). It is a hormone: a messenger the body makes. This one helps a girl's body change into a woman's body, and it helps run the monthly cycle. The tidy classroom line is 'the female hormone,' and that is how most people use it, though the messy truth is kinder and wider. There is a small family of these chemicals, not one molecule; men make some too, just much less; and doctors also give it as medicine when the ovaries make little of it. You will meet it in health class, on medicine boxes, and in news about the body. It is a science word. Friends do not toss it around at dinner unless they are talking about health.