Longman Dictionary of Contemporary English - unless

~ conjunction used when one thing will only happen or be true as long as another thing happens or is true  (Milk quickly turns sour unless it's refrigerated. | Unless some extra money is found, the theatre will close.)  ( USAGE NOTE: UNLESS WORD CHOICE unless, if...not, if, provided/providing that You do not use unless about something that did not happen or you know is not true, but you can use if...not in this way. For example, you could not use unless in these sentences She would have died if the doctors hadn't operated immediately (=but they did).| If he wasn't such an idiot (=but he is) he would understand. You use unless about something that could happen if something else does not happen or is not done, or if something could be true Unless the doctors operate immediately, she'll die. | She'll die unless the doctors operate immediately (=the doctors have not operated yet, and may or may not do).| Unless he's a complete idiot, he'll understand (=he may or may not be a complete idiot). Unless and if...not can both be used to say that what you will do depends on something else happening Unless he comes soon, I'm going without him. | If he doesn't come soon, I'm going without him. You use provided (that) or providing (that) when the condition is that something definitely happens or is definitely true Providing the weather's OK, we'll make the trip next Saturday. You often use unless to add a condition that you think of after you have finished speaking We'll go there next Saturday - unless it's raining of course. )