A molecule with an affinity to bind to a second atom or molecule. This affinity can be described in terms of noncovalent interactions, such as the type of binding that occurs in enzymes that are specific for certain substrates; or of a mode of binding where an atom or groups of atoms are covalently bound to a central atom, as in the case of coordination complexes and organometallic compounds. Ligands of the latter type can be further distinguished by the nature of the orbitals used in bond formation.