synonymy

Near or identical meaning between or among words. There are very few strict synonyms in any language, although there may be many near-synonyms, depending upon the contexts in which the words are used.

Thus brotherly and fraternal are synonyms in English, but a brotherhood is not the same as a fraternity. People talk about the brotherhood of man but seldom if ever about the ‘fraternity of man’. Brotherhood and fraternity are not therefore strictly synonymous.