A molecule that functions as a message within an organism; its only function is to convey information.
Because of this function, physical descriptions of a chemical suspected of being a hormone are inadequate to indicate the molecule's role in a biological system. A molecule is a hormone only when described in the context of its role in a biological communication system. Definition of a hormone requires testing of that molecule in a biological response system, running a bioassay.
The existence of endocrinology is totally dependent on the existence and use of bioassays. (This is also true for pharmacology and toxicology.)