(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.

(linguistics, philosophy) A distinction between the use of a word for its meaning (as in "Cheese is derived from milk") and the mention of a word as a lexical unit (as in " 'Cheese' is derived from a word in Old English").

(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.