ligand

"Any molecule that binds tightly and specifically to a macromolecule, usually a protein, forming a macromolecule-ligand complex." (source)

Examples:

Acetylcholine is a ligand that binds to the acetylcholine receptor (a ligand-gated ion channel).

In receptor-mediated endocytosis, IgA binds to the poly-Ig receptor.

Ivabradine is a drug that binds to the funny channel.

Cardiac glycosides bind to the Na⁺,K⁺ pump.

Diagram (below): receptor binding specificity.

Receptor binding sites only tightly bind ligands that have a shape that is complementary to the structure of the receptor binding site (lock and key model). In the diagram, two different receptor proteins are shown. Only one will bind the ligand.