A nerve fiber that terminates on a muscle or gland and stimulates contraction or secretion WordNet 3.0 & Open English WordNet

In biology, an effector is a general term that can refer to several types of molecules or cells depending on the context: Small molecule effectors A small molecule that selectively binds to a protein to regulate its biological activity can be called an effector. Wikipedia

A molecule that binds to a protein and thereby alters the activity of that protein Wikipedia Disambiguation

Small molecule affecting biological activity Wikidata

Any small molecule that effects the function of an enzyme by binding to an allosteric site. Wiktionary