A subalgebra of a given algebra is a SubSet thereof CloseD? under all relevant operations, so that it forms an algebra of the same kind under them (or more technically their restrictions). A MappinG between two algebrae that preserves all relevant operations and relations is called a HomoMorphism; if it is also a BiJection, it is called an IsoMorphism?. Isomorphic algebrae are indistinguishable as far as algebraic structure is concerned.