unless

Except on a specified condition; if not.

I’m leaving unless I get a pay rise.

You can't go out unless it stops raining.

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?

And Joab said, As God liveth, unless thou hadst spoken, surely then in the morning the people had gone up every one from following his brother.

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.

Lie down and finish out the night, since you are here; but, for heaven's sake! don't repeat that horrid noise: nothing could excuse it, unless you were having your throat cut!