Also found in: Dictionary.
1. If (something) is not true or not the case. Do you want that last slice of pizza? If not, I'll take it. Did they call yet? If not, I'll wait to get in the shower. I'll go to a movie if you guys want to. If not, I'll probably just go to bed.
2. A phrase used to emphasize that something may be even more extreme than originally indicated. I haven't had a good night's sleep in weeks if not months, but that's life with a newborn, I guess.
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.