What is the meaning of Adjunction?

The act of joining; the thing joined or added.

The joining of personal property owned by one to that owned by another.

Given a pair of categories and : an anti-parallel pair of functors and and a natural transformation called “unit” such that for any object , for any object , and for any morphism , there is a unique morphism such that . (Note: there is another natural transformation called “counit” as well but its existence may be derived by theorem.) The pair of functors express a similarity between the pair of categories which is weaker than that of an equivalence of categories.

Michael Barr with Charles Wells (1995) Category Theory for Computing Science, second edition, University Press, Cambridge, Great Britain: Prentice Hall, →ISBN, §9.2, page 258

Source: wiktionary.org