1) One of various structurally related forms of an enzyme, each having the same mechanism but with differing chemical, physical, or immunological characteristics.

2) different form of the same enzyme, such as from another tissue or species, membrane-bound vs. soluble, secreted vs. intracellular, or oligomerized to a different degree.

3) Any of two or more chemically distinct but functionally similar enzymes -- called also isozyme.