1. Any individual atom, group, or molecule attached to a central metal ion by multiple coordinate bonds, the porphyrin portion of heme, the corrin nucleus of the B12 vitamins. 2. An organic molecule attached to a tracer element, a radioisotope. 3. A molecule that binds to a macromolecule, a ligand binding to a receptor. 4. The analyte in competitive binding assays, such as radioimmunoassay. 5. An atom or group covalently attached to a specified carbon atom in an organic molecule.