agonist-1) In pharmacology (drug study), a drug that has has the property of affinity and stimulates physiologic (normal functional) activity in cells normally only stimulated by some naturally occuring substance in the body.

You probably hear of people who use agonists everyday without taking notice. Many people who use inhalers for asthmatic symptoms, are using agonists Drugs like salbutamol are agonists, because they stimulate cells that normally are only stimulated by chemicals made in the human body. This "artificial stimulation" is beneficial since the agonist relaxes bronchial smooth muscle, restoring normal unlaboured breathing.

2) In anatomical terms, a prime mover.