A category with finite products and coproducts is called (finitary) distributive if for any the canonical morphism
is an isomorphism. The canonical morphism is the unique morphism such that is , where is the coproduct injection, and dually for .
This axiom on binary coproducts easily implies the analogous -ary result for . In fact it also implies the analogous 0-ary statement that the projection
is an isomorphism for any . Moreover, for a category with finite products and coproducts to be distributive, it actually suffices for there to be any natural family of isomorphisms , not necessarily the canonical ones; see the paper of Lack referenced below.
A category with finite products and all small coproducts is infinitary distributive if the statement applies to all small coproducts. One can also consider -distributivity for a cardinal number , meaning the statement applies to coproducts of cardinality .
Any extensive category is distributive, but the converse is not true.
Carboni, Aurelio and Lack, Stephen and Walters, R. F. C., Introduction to extensive and distributive categories, JPAA 84 no. 2
Stephen Lack, Non-canonical isomorphisms. arXiv:0912.2126.