(set phrase) A linguistic or conceptual distinction which is of no practical importance or which has no effect on meaning; a perceived difference where there is no actual difference.

(education) The distinction drawn by some language teachers between an "error" (a mistaken belief that a given usage or construction is correct; or, an incorrect use resulting from such a belief) and "a mistake" (an accidental use of a usage or construction despite knowing that it is incorrect).

(sociolinguistics) The distinction between a formal and an informal second-person pronoun in some languages.

plural of distinction without a difference