Also found in: Dictionary.
if that is not [the case]; if that is not so. If not cleaned, the clock will stop running. He should be here at noon. If not, we will eat without him.
1 used to introduce a different suggestion, after a sentence with if : I’ll go if you’re going. If not (= if you are not) I’d rather stay at home.
2 used after a yes/no question to say what will or should happen if the answer is ‘no’: Are you ready? If not, I’m going without you. ♢ Do you want that cake? If not, I’ll have it.
3 used to suggest that something may be even larger, more important, etc. than was first stated: They cost thousands if not millions of pounds to build.