Myoglobin (blue) with its ligand heme (orange) bound.

A ligand is a biochemical substance. Its function is to bind to a biological molecule, and make the molecule perform a function. Ligand binding alters the shape of the receptor protein. This changes the protein's behaviour.

In a narrower sense, it is a signal triggering molecule, binding to a site on a target protein.

The binding occurs by intermolecular forces, such as ionic bonds, hydrogen bonds and van der Waals forces. The docking (association) is usually reversible (dissociation).