Define what Trotskyism is.

Answer: the political, economic, and social principles advocated by Trotsky

(especially : the theory and practice of communism developed by or associated with Trotsky and usually including adherence to the concept of worldwide revolution as opposed to socialism in one country)

Leon Trotsky's political or economic concepts, in particular the idea that the ongoing movement would create socialism all over the world. In fact, Trotskyism contained aspects of anarchism and syndicalism, but the concept has been used indiscriminately to describe other types of democratic socialism.