not Definition

not
adverb

1. Used to make a negative statement, etc

Example: That is not fair
Example: Those aren't right
Example: Why didn't they come?
2. Used with verbs of opinion or intention, etc to make the clause or infinitive following the verb negative, as in I don't think he's right, meaning I think he is not right.
3. Used in place of a negative clause or predicate.

Example: might be late, but I hope not
4. (indicating surprise or an expectation of agreement, etc) surely it is the case that

Example: Haven't you heard?
Example: Lovely, isn't it?
5. Used to contrast the untrue with the true.

Example: It's a cloud, not a mountain
6. Barely.

Example: with his face not two inches from mine
7. By no means.

Example: not nearly enough
Example: Not everyone would agree
8. colloq
Used by a speaker to emphatically deny what they have just said
Example: looks a lot like Brad Pitt…not.

Idiom: not a ...

Absolutely no ....
Example: not a sound

Idiom: not at all

Don't mention it; it's a pleasure.

Idiom: not just (not only)

Used to introduce what is usually the lesser of two points, etc.
Example: not just his family, but his wider public

Idiom: not on

Not possible.

Not morally or socially, etc acceptable.

Idiom: not that ...

Though it is not the case that ....
Example: not that I care

Etymology: 14c: a variant of nought.