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
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 then already an expression of weakness that reproduced in the First International and massive political structures (1st and 2nd Internationals) the old messianic and quasi-religious forms of utopianism
The development of socialist sectarianism and the development of the real labor movement are always in inverse proportion. Sects are (historically) justified as long as the working class has not yet matured into an independent historical movement. But as soon as it has reached that maturity, all sects become essentially reactionary. By the way, the history of the International has repeated what general history shows us everywhere. The outdated tends to re-establish itself and to maintain its positions within the newly reached forms. The history of the International has also been a continuous struggle of the General Council against the sects and the experiments of dilettantes which tended to take root in the International against the real movement of the working class.
Karl Marx. Letter to Frederick Bolte, 23 November 1871
The Second International knew sectarianism in the form of "organizational patriotism", that is to say, 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 by zinovievism first and by stalinism later. That is to say, it was used by the political expressions of the degeneration and capture of the International.http://dictionary.marxismo.school/Sectarianism