Unless

//ənˈlɛs// conj

Definitions

Conjunction
  1. 1
    Except on a specified condition; if not.

    "I’m leaving unless I get a pay rise."

  2. 2
    If not; used with counterfactual conditionals.

    "Who would know of Alexander or Caesar, or would respect the Stoics or the Peripatetics, unless they had been distinguished by the memorials of writers?"

  3. 3
    Except if; used with hypothetical conditionals.

    "Q. If Lieutenant P. had given the word "Fire," would you have fired, and at what?—A. I should not have known, unless he had told me what to fire at."

Etymology

From Middle English unlesse, earlier on lesse (modern on + less), from (not) on lesse (than) (“(not) on a less compelling condition (than)”). The quality of negation in the word and the lack of stress changed on to un-.

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