noun. 1. a substance which is utilized in conjunction with another substance in effort to remedy an illness and render a surplus of other medicinal effects. It is possible that a system of behaviors, varying from the behaviors of the origin substance, could be present. 2. a keyword or reference within a sentence which isn't a primary constructive component part but rather one which changes, increases, or features an alternate component part which is. 3. even more commonly, a secondary or inessential component of anything.