Historical revisionism is a type of research that aims to change the conventional, most widely-accepted understanding of specific historical events. History is, after all, written by the winners. Historical revisionism thus is legitimate historical research; however, this term also, perhaps unfortunately, is now associated with Holocaust revisionism, a denial that the Holocaust took place. Most of the people who claim that the Holocaust never took place refer to themselves as historical revisionists, in order to sanitize their views. Much of the public, however, is unaware of legitimate historical revisionism, and thus this term is now sometimes used to refer to historical revisionists. When using (or reading) this phrase, make sure the meaning of the word is clear from context.
Individual historical revisionist theories are known as revisionist histories.
Historical revisionism is necessarily controversial, as it challenges widely-held beliefs. Evidence supporting revisionist histories is generally sparse or poorly documented. Belief in such histories are often based in belief of a conspiracy theory to explain a lack of documentation. An example is the theory that Franklin Roosevelt had prior knowledge about the attack on Pearl Harbor, and allowed it to happen.
Some revisionist histories have become the generally accepted history as more information is revealed; for example, many history books of the past rarely mentioned, if at all, the relationship the European explorers, colonists, and later the United States had with the Native American population (who were referred to as American Indians or Red Indians). In the past, outside of Native American populations, very few would dispute the assertion that Christopher Columbus discovered America. As another example, throughout history slaves have not been considered as people, which has been reflected in the accepted histories of the time.