- Not to be confused with Historical revisionism
Revisionism is a fundamental alteration of a theory, essentially usurping (though taking elements of) the former theory and replacing it with a new one. While the attributes of a theory are subject to change in accordance to changing historic circumstances, changing the fundamental basis of that theory is to nullify it in place of a new one.
Revisionist theory, politics and praxis are only so when examined through the lens of dialectical materialism and found incorrect in one of two ways: either contradicting past successes, such as the vanguard party, which was proven successful with the Bolsheviks, Communist Party of Vietnam, and the Communist Party of China, or by pushing errors with a history of failure and incoherence. The Narodniks were unable to cohesively grasp that the feudal landlord and peasant ends with the capital production of agriculture.
Eduard Berstein, infamously noted in Lenin's "What is to be done", was a leading theoretician of the German Social Democratic Party who originated the revisionist theory of evolutionary socialism. He rejected Marxist concepts of class struggle and world revolution, liquidating Marxism and removing its revolutionary elements to focus entirely on reformism.
Left & Right Deviation
Trotskyism and Maoism are often described within Marxism Leninism circles as left deviations that do not engage in sufficient criticism and self-criticism and thus fall off the path of scientific socialism. In parallel, reformism is a right deviation that cedes one of the fundamental aims of Marxism: the abolition of and replacement by proletarian structures regarding the bourgeois control over the means of production.