- A substance from which an enzym may be formed by internal change. Also <internalXref urlencoded="zymogene">zymogene</internalXref>.
- A mother substance, or antecedent, of an enzyme or chemical ferment; -- applied to such substances as, not being themselves actual ferments, may by internal changes give rise to a ferment.
- A <xref>proenzyme</xref>, or <xref>enzyme</xref> <xref>precursor</xref>, which requires a biochemical change (i.e. hydrolysis) to become an active form of the enzyme.
- any of a group of compounds that are inactive precursors of enzymes and require some change (such as the hydrolysis of a fragment that masks an active enzyme) to become active