An agonist is a ligand that binds to a receptor and stabilises the receptor conformation associated with signal transduction, producing a functional response of the same type as the endogenous ligand. In classical pharmacology a full agonist is defined as producing the maximum response the system can generate (intrinsic efficacy = 1).
Agonists are characterised by two independent parameters: affinity, which describes how tightly they bind, and efficacy, which describes how effectively binding translates into a signal. A dose-response curve for an agonist is typically summarised by its EC50 (a potency measure) and its maximal response (an efficacy measure).
Many research peptides — GLP-1, GHSR and melanocortin ligands — are studied precisely because they act as agonists at defined receptors.