18XX 


Category theory; homological algebra {For commutative rings see 13Dxx, for associative rings 16Exx, for groups 20Jxx, for topological groups and related structures 57Txx; see also for algebraic topology} 

18Dxx 

Categories with structure 


18D05 
Double categories, $2$categories, bicategories and generalizations 


18D10 
Monoidal categories (= multiplicative categories), symmetric monoidal categories, braided categories [See also 19D23] 


18D15 
Closed categories (closed monoidal and Cartesian closed categories, etc.) 


18D20 
Enriched categories (over closed or monoidal categories) 


18D25 
Strong functors, strong adjunctions 


18D30 
Fibered categories 


18D35 
Structured objects in a category (group objects, etc.) 


18D50 
Operads [See also 55P48] 


18D99 
None of the above, but in this section 