unless

conjunction3

1. •
   except if, used to name the only condition under which something else does not happen or is not true

   • I’m leaving unless I get a pay rise.
   • You can't go out unless it stops raining.
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? — 1990
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. — 1809