Ligand

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). 
Ligands include substrates, inhibitors, activators, and neurotransmitters. The tendency or strength of binding is called affinity.