1. Any of a very large class of complex proteinaceous substances (as amylases or pepsin) that are like catalysts in promoting at the cell temperature usu. reversible reactions (as hydrolysis and oxidation) without themselves undergoing marked destruction in the process but frequently requiring the presence of activators (as metal ions) or of coenzymes, and that can act also outside of living organisms and therefore are useful in many industrial processes (as fermentation, tanning of leather, and production of cheese.
ref: Webster's International Unabridged Dictionary