Quasi-isomorphism

In homological algebra, a branch of mathematics, a quasi-isomorphism or quism is a morphism "A" → "B" of chain complexes (respectively, cochain complexes) such that the induced morphisms 
of homology groups (respectively, of cohomology groups) are isomorphisms for all "n".
In the theory of model categories, quasi-isomorphisms are sometimes used as the class of weak equivalences when the objects of the category are chain or cochain complexes. This results in a homology-local theory, in the sense of Bousfield localization in homotopy theory.