Organic molecules (cf. coenzymes) or ions (usually metal ions) that are required by an enzyme of its activity. They may be attached either loosely or tightly prosthetic group) to the enzyme. A cofactor binds with its associated protein (apoenzymes), which is functionally inactive, to form the active enzyme (holoenzyme).
PAC, 1992, 64, 143 (Glossary for chemists of terms used in biotechnology (IUPAC Recommendations 1992)) on page 150