Ultra is short for Ultra Left, which very broadly speaking can be considered a form of what Lenin called “Left-Wing” Communism.

There are a lot of different kinds of ultras, and depending on who you ask you will get a different answer as to who the label applies to (Anarchists, Trotskyists, Maoists, etc.), but what generally unites them all is their idealism, their dogmatism, and their focus on ideological purity. They reject materialist analysis that doesn’t conform with their ideology and demand the skipping of stages in the development of socialism. Ultra-leftism is often associated with adventurism and hyper-militancy.

Ultra-leftism in many ways is simply the opposite side of the same coin to which right-opportunism belongs. They are undialectical deviations from Marxism. Because of their shared lack of solid dialectical grounding, sometimes former ultra-leftists, feeling scorned or frustrated, can flip straight into becoming right deviationists (capitalist roaders), or worse. Their danger stems from the fact that both can steer revolutionary forces and socialist governments off-course and into dead ends, though they do so in different ways.

“It should not be forgotten that Rights and “ultra-Lefts” are actually twins, that consequently both take an opportunist stand, the difference between them being that whereas the Rights do not always conceal their opportunism, the Lefts invariably camouflage their opportunism with “revolutionary” phrases.”

The Fight Against Right and “Ultra-Left” Deviations, - J. V. Stalin