1The result of being made equal, balanced or commensurate; equivalence; exact correspondence; an equivalent. Frequently, especially in early use, in philosophical contexts with reference to truth.
2The action of making commensurate, equal, or balanced; the matching or proportioning of one thing to another.
In or with reference to the theories of Gustaf Stern: a type of semantic change arising from a change in the perception of a word's referent, e.g. the word horn coming to refer to a wind instrument as this became the predominant use for animal horns.
Late 16th century (in an earlier sense). From post-classical Latin adaequation-, adaequatio agreement (late 2nd cent. in Tertullian), action or fact of balancing or levelling, correspondence from classical Latin adaequāt-, past participial stem of adaequāre + -iō.