1. animal (insect) hormone a chemical substance formed in the endocrine system that is secreted internally into the body fluids and that influences or activates specifically receptive organs or physiological processes 2. plant hormone an organic plant compound such as auxin or gibberellin that in minute (trace) quantities, remote from where it is formed, controls growth and differentiation of plant tissue —see growth regulator, pheromone, semiochemicalThis definition last updated 10/28/2008