is in iff and are in .
Following Serre, one then defines the category whose objects are the objects of and where the morphisms in are defined by
where the colimit is over all in such that and are in . The quotient category is abelian.
Very often the same definition of Serre subcategory is used in an arbitrary abelian category (we will say in that case weakly Serre subcategory); but in fact, at least when the abelian category is not a Grothendieck category, it is more appropriate to ask for an additional condition in the definition of Serre subcategory, so that the standard theorems on correspondences with other canonical data in localization theory remain valid.
To this aim, for any subcategory of an arbitrary abelian category one denotes by the full subcategory of generated by all objects for which any (nonzero) subquotient of in has a (nonzero) subobject from . This becomes an idempotent operation on the class of subcategories of with iff is topologizing. Moreover is always thick in the stronger sense (that is, thick and topologizing).
Serre subcategories in the strong sense are those nonempty full subcategories which are stable under the operation .