Organizational tendency that puts the particular interests and achievements of an organization above the general interests of the class movement and its historical perspective.
Origins and evolution of sectarianism
In the origins of the labor movement, the "socialist sects" saw themselves, and not the class, as the subject called upon to transform reality. They were already then an expression of weakness that reproduced in the first international mass political structures (1st and 2nd Internationals) the old messianic and quasi-religious forms of utopianism.
The development of socialist sectarianism and that of the real labor movement always stand in inverse ratio to each other. Sects are justified (historically) so long as the working class is not yet ripe for an independent historical movement. As soon as it has attained this maturity, all sects become essentially reactionary.
Incidentally, the history of the International has repeated what history in general shows us everywhere. What is obsolete tries to re-establish itself and maintain its positions within the newly achieved forms. The history of the International has also been a continuous struggle of the General Council against the sects and the amateur experiments which tried to assert themselves within the International against the real movement of the working class.
Karl Marx. Letter to Friedrich Bolte, November 23, 1871
The Second International knew sectarianism in the form of "organizational patriotism", that is, as a tendency to deny debate and critical analysis of one's own positions when they clashed with reality, so as not to "weaken confidence in the organization". This type of sectarianism also occurred in the Third International and was used instrumentally first by Zinovievism and later by Stalinism. That is to say, it was used by the political expressions of the degeneration and capture of the International.