Opportunism
"In one way or another, all these petty-bourgeois groups penetrate into the Party and introduce into it the spirit of hesitancy and opportunism, the spirit of demoralization and uncertainty. It is they, principally, that constitute the source of factionalism and disintegration, the source of disorganisation and disruption of the Party from within. To fight imperialism with such "allies" in one's rear means to put oneself in the position of being caught between two fires, from the front and from the rear. Therefore, ruthless struggle against such elements, their expulsion from the Party, is a pre-requisite for the successful struggle against imperialism."
— Joseph Stalin, Foundations of Leninism
Opportunism is collaboration of communists with the bourgeoisie, unlike class collaborationism which is collaboration of proletarians with the bourgeoisie. An example of opportunism would be socialists collaborating with revisionists or anti-Stalinists.