1. A protein molecule produced by living organisms that catalyses chemical reactions of other substances without itself being destroyed or altered upon completion of the reactions: Doug learned about the importance of enzymes in that they were necessary for breaking down large molecules of fat, starch, and protein during the process of digestion.
2. A protein, or protein-based molecule, that speeds up a chemical process in a living organism: An enzyme acts as catalyst for specific chemical reactions, converting a specific set of reactants, called substrates, into specific products. Without enzymes, life as we know it would not exist.
3. Any of numerous proteins or conjugated proteins produced by living organisms and functioning as biochemical catalysts: There are at least 66 types of enzymes, including cholinesterase, coagulase, and histamines.

Enzymes are classified according to the recommendations of the Nomenclature Committee of the International Union of Biochemistry.

Each enzyme is assigned a recommended name and an Enzyme Commission (EC) number.

Enzymes are divided into six main groups, oxidoreductases, transferases, hydrolases, lyases, isomerases, and ligases.