Real Dictionary

hormone
countable noun: A hormone is a chemical, usually occurring naturally in your body, that makes an organ of your body do something.

hormone in American English
a substance formed in some organ of the body, as the adrenal glands, the pituitary, etc., and carried by a body fluid to another organ or tissue, where it has a specific effect: now often prepared synthetically
noun: any of various internally secreted compounds, as insulin or thyroxine, formed in endocrine glands, that affect the functions of specifically receptive organs or tissues when transported to them by the body fluids

hormone in British English
noun: a chemical substance produced in an endocrine gland and transported in the blood to a certain tissue, on which it exerts a specific effect