What is the difference between a substrate, ligand, reactant and product?

A ligand , in biology, is a molecule that binds to another. Often, a soluble molecule such as a hormone or neurotransmitter that binds to a receptor. Here growth factor efficacy is increased with ligand/receptor binding.

Ligand, in chemistry, any atom or molecule attached to a central atom, usually a metallic element, Here copper is the ligand.

The word is used in ligature (something that binds two or more thing together). Actually handcuffs are ligatures.

A substrate is a molecule upon which an enzyme acts. The substrate is changed by the reaction and, in this case, two products are made.

A reactant and substrate have the same meaning. The term reactant is more often used in chemistry.