> 18-XX Category theory; homological algebra > 18CXX Categories and theories

18C05 Equational categories	18C10 Theories (e.g. algebraic theories), structure, and semantics	18C15 Triples (= standard construction, monad or triad), algebras for a triple, homology and derived functors for triples
18C20 Algebras and Kleisli categories associated with monads	18C30 Sketches and generalizations	18C35 Accessible and locally presentable categories
18C50 Categorical semantics of formal languages	18C99 None of the above, but in this section

18CXX Categories and theories