Let’s start our duty with a « common sense » definition of negationism found on Wikipedia. There can be a debate about it !
For the legitimate re-examination of history, see historical revisionism
Negationism, as it applies to Historical revisionism, is the denial of historic crimes. The word is derived from the French term négationnisme, which means denial, which is illegal in France and several other countries.
In attempting to revise the past, negationism appeals to the intellect—via techniques illegitimate to historical discourse—to advance a given interpretive historical view, typically involving war crimes or crimes against humanity. The techniques include presenting known forged documents as genuine; inventing ingenious, but implausible, reasons for distrusting genuine documents; attributing his or her own conclusions to books and sources reporting the opposite; manipulating statistical series to support the given point of view; and deliberately mis-translating texts (in languages other than the revisionist’s). Practical examples of negationism (illegitimate historical revisionism) include Holocaust denial and some Soviet historiography.
Contemporarily, hate groups practice negationism on the Internet. In literature, the effects of historical revisionism are usually described in science fiction novels such as Nineteen Eighty-Four (1949), by George Orwell. Moreover, some countries have criminalised the negationist revision of certain historical events.