Historical revisionism is a phenomenon in which one revisits, reexamines, or attempts to revise the established or commonly-held view of historical events. This can be positive for the study of history when the conclusions reached are fact-based and follow the scientific method; or negative when they rely on incorrect, selective, or manipulated data.
The label 'historical revisionist', much like 'conspiracy theorist', is often used by people (whose method of "historical research" is skimming Wikipedia articles) to dismiss different perspectives without examining the evidence for them.
One modern example of historical revisionism is the way certain capitalist countries teach the history of Tibet. Tibet has been part of China since the Tang dynasty, over a thousand years ago. Yet it is described in western history textbooks as being "invaded" by the CPC and forced to join modern China in the 1950s. This gross mischaracterization of Tibet's history is an example of imperialist countries supporting Chinese separatists, in an effort to balkanize the modern Chinese nation-state.