Except in so far as some familiar enzymes continue to be known by their old established names (pepsin, rennin, trypsin, etc.), scientific usage now generally follows the suggestion of Duclaux that each hydrolytic enzyme be designated by a name indicating the kind of substance on which it acts, together with the suffix ase. Thus starch-splitting enzymes are called amylases; fat-splitting enzymes, lipases; protein-splitting enzymes, proteases. The name showing the activity of the enzyme is often preceded by an adjective to indicate its source; e.g. salivary amylase (ptya-lin), pancreatic amylase (amylopsin). Such designation does not necessarily imply that the amylase found in the saliva either is or is not the same substance as the amylase of the pancreatic juice.

In discussions of enzyme action the substance on which the enzyme acts is sometimes called the substrate.

Within the cell producing it an enzyme often exists in an inactive form known as the zymogen or antecedent of the active enzyme. The zymogen may be stored in the cell in the form of material which is converted into active enzyme at the time of secretion, or the secretion may be poured out with the zymogen not yet completely changed to active enzyme, or sometimes in a form which requires the presence of some other substance in order to render it active. In this case the latter substance is said to activate the enzyme.