Marxist-Leninist. To simplify, Marx wrote some stuff, Lenin expanded on it and threw a revolution, shortly after which he fucking died. Stalin wrote a couple more sentences then distilled the ideology of Marxism-Leninism. That’s why you have non-Marxist-Leninist Marxists/Leninists like the ICP, as word-salady as that sounds.
The essential factor is the belief in the use of a Vanguard Party to carry out a revolution and build up a Socialist state to whither into Communism.