Ligand. Definition. noun, plural: ligands. (1) A molecule, ion or atom bonded to the central metal atom of a coordination compound. (2) Any substance (e.g. hormone, drug, functional group, etc .) that binds specifically and reversibly to another chemical entity to form a larger complex.Supplement. A ligand may function as agonist or antagonist.1) A molecule, ion or atom bonded to the central metal atom of a coordination compound. (2) Any substance (e.g. hormone, drug, functional group, etc .) that binds specifically and reversibly to another chemical entity to form a larger complex.