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The Collaborative International Dictionary of English v.0.48 (gcide)

Hormone \Hor"mone\ (h[^o]r"m[=o]n), n. [From Gr. "orma`ein to
excite.]
1. (Physiological Chem.) A chemical substance formed in one
organ and carried in the circulation to another organ on
which it exerts a specific effect on cells at a distance
from the producing cells; thus, pituitary hormones
produced in the brain may have effects on cells in distant
parts of the body..
[Webster 1913 Suppl. PJC]

2. (Physiological Chem.) a chemical substance, whether
natural or synthetic, that functions like a hormone in a
living organism. Thus, synthetic steroid hormones may be
more effective than their natural counterparts.
[PJC]

3. (Bot.) A substance that controls growth rate or
differentiation in plants; also called {phytohormone}. The
most well-known are the {auxins} that stimulate growth at
the growing tips of plants, and control root formation and
the dropping of leaves; and the {gibberellins}, which are
used in agriculture to promote plant growth.
[PJC]