18C05 Equational categories [See also 03C05, 08C05]
18C10 Theories (e.g. algebraic theories), structure, and semantics [See also 03G30]
18C15 Triples (= standard construction, monad or triad), algebras for a triple, homology and derived functors for triples [See also 18Gxx]
18C20 Algebras and Kleisli categories associated with monads
18C30 Sketches and generalizations
18C35 Accessible and locally presentable categories
18C50 Categorical semantics of formal languages [See also 68Q55, 68Q65]
18C99 None of the above, but in this section