adjunctions

Noun

adjunction (countable and uncountable, plural adjunctions)

1. The act of joining; the thing joined or added.
2. The joining of personal property owned by one to that owned by another.
3. The process of adjoining elements to an algebraic structure (usually a ring or field); the result of such a process.
   • The ring obtained after the adjunction of the elements a,b and y to the ring R may be denoted R#91;a,b,y#93;.
4. (broadly) A relationship between a pair of categories that makes the pair, in a weak sense, equivalent.
5. (strictly) A natural isomorphism between a pair of functors satisfying certain conditions, whose existence implies a close relationship between the functors and between their (co)domains; the natural isomorphism, functors, and their (co)domains thought of as a single object.
   1. (formal, strictly) A natural isomorphism Φ: operatorname Hom_( mathcal )C(G·,·)→ operatorname Hom_( mathcal )D(·,F·) (where the hom-functors are understood as bifunctors from 𝒟^( operatorname )op×𝒞 to mathbf Set). See Adjoint functors on Wikipedia.Wikipedia .