1. Pada anatomi, penggerak utama.
  2. Dalam farmakologi, suatu obat yang mempunyai afinitas terhadap dan merangsang aktivitas fisiologis reseptor sel yang biasanya dirangsang oleh substansi yang secara alami terdapat di situ.
Merriam-Webster Online Dictionary
agonist (noun)
one that is engaged in a struggle
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 specific receptor on a cell and initiating the same reaction or activity typically produced by the binding endogenous substance - dopaminergic agonists compare antagonist 2b
Agonist (Wikipedia)
For other meanings of "agonist" or "agonism", see agonist (disambiguation). For a primary fictional character in a work, see Protagonist and Antagonist.
Agonists activating hypothetical receptors.
Efficacy spectrum of receptor ligands.

An agonist is a chemical that binds to a receptor and activates the receptor to produce a biological response. Whereas an agonist causes an action, an antagonist blocks the action of the agonist and an inverse agonist causes an action opposite to that of the agonist.