Not is defined as a word used with a verb to make a negative.(adverb)
An example of not used as an adverb is in the phrase "not happy," which means unhappy.
See not in Webster's New World College Dictionary
Origin: ME not, unstressed form of noht, nought, naught: see nought
See not in American Heritage Dictionary 4
Origin: Middle English
Origin: , alteration of naught, nought; see naught. Usage Note: Care should be taken with the placement of not and other negatives in a sentence in order to avoid ambiguity. All elephants are not friendly could be taken to mean either “All elephants are unfriendly” or “Not all elephants are friendly.” Similarly, the sentence Kim didn't sleep until noon could mean either “Kim went to sleep at noon” or “Kim got up before noon.” • In formal writing, each part of the not only . . . but also construction should be followed with an element of the same grammatical type. Instead of She not only bought a new car but also a new lawnmower, one should write She bought not only a new car but also a new lawnmower; in the second version, both not only and but also are followed by noun phrases. Omitting the also tends to intensify the second part of the construction so that it no longer functions merely as a supplement to the first part: She is not only smart but brilliant. He not only wanted the diamond but wanted it desperately. See Usage Note at only.
Learn more about not