Historical revisionism is the phenomenon in which one revisits, reexamines, and attempts to revise the established or commonly-held view of historical events. This can be helpful to the study of history when the conclusions reached are fact-based and follow the scientific method; or harmful 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 said perspectives.