[ag.o.nist] n [LL agonista competitor, fr. Gk agonistes, fr. agonizesthai to contend, fr. agon] (ca. 1623) 1: one that is engaged in a struggle
2. [fr. antagonist] a: a muscle that is controlled by the action of an antagonist with which it is paired b: a chemical substance capable of combining with a receptor on a cell and initiating a reaction or activity--compare antagonist 2b