adjunction

noun

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.
4. A relationship between a pair of categories that makes the pair, in a weak sense, equivalent.
5. 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.
6. 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 .