Learner's definition of NOT
1 a — used to form the negative of modal verbs (such as “should” and “could”) and auxiliary verbs (such as “do” and “have”)
b — used before a verb or clause to make it negative or give it an opposite meaning
He would/could not stay.
We have not spoken with them.
It may not be fast, but it's reliable.
That kind of behavior should not be allowed.
He did not seem to care.
They gave us the option of not attending.
She told me not to do it.
You are not to go there without permission.
He seemed not to care.
It's odd, but that's not to say [=that does not mean] that it's bad.
2 a — used with a word or phrase to make it negative or give it an opposite meaning
The books are not here.
No, that's not what I said.
That's not funny!
That is not necessarily true.
You're not always right.
You are not the only one who is worried.
She is not (at all/very) pleased/happy.
I'm not happy or sad, just bored.
It is not as easy as it seems.
Things are not going well.
He is not guilty.
He lives not far from here.
“Are we there yet?” “Not quite.”
“Is there any left?” “Not much.”
It is not just/only/merely [=more than just] a novel; it is a literary classic.
Not many [=few] people showed up.
b — used with a negative word to make a positive statement
— see also not a few at 1few
c informal + humorous — used humorously at the end of a positive statement to show that you really meant the opposite
Their request is not unreasonable. [=their request is reasonable]
His experience is not unusual. [=others have had similar experiences]
“What do you think of the food?” “It's not bad.” [=it's pretty good]
3 : less or fewer than
He was standing not six feet away from me.
The bullet passed not five inches from my head!
Not all of us agree. [=some of us do not agree]
Not everybody finds her work interesting. [=some people find her work boring]
4 — used to refer to a possible situation, condition, etc., that is different from or opposite to another situation, condition, etc. — often used in the phrase or not
Believe it or not [=whether you believe me or don't believe me], we were there just last week.
Ready or not, here I come!
Like it or not, you'll do as he says. [=you'll do as he says whether you like or don't like it]
5 — used to give a negative answer to a question
“Do you think they forgot?” “I hope not.”
“Should we go?” “I'd rather not.”
“Do you think it's true?” “Probably/Certainly not.”
“Do you mind?” “Not at all.”
as like as notorlike as not
more often than not
: happening more than half the time
He wins more often than not. [=he wins more than he loses]
More often than not, I stay home instead of going out.
: no thing or person
There was not one available taxi to be found. [=no taxis were available]
Not a/one single person showed up. [=no one showed up]
Not a word came out of her mouth.
— used to say that something said before is not important
I tried to help, not that it mattered. [=but it did not matter]
I saw him with some other woman, not that I care. [=but I don't care]