The non-amino acid portion of a conjugated protein. Examples include cofactors such as flavines or cytochromes as well as lipids and polysaccharides, which are the prosthetic groups of lipoproteins and glycoproteins, respectively.
Source:
PAC, 1992, 64, 143. 'Glossary for chemists of terms used in biotechnology (IUPAC Recommendations 1992)' on page 163 (https://doi.org/10.1351/pac199264010143)