Remind me to come back to this later with a link to a book that I've been reading that contextualizes criticisms of Stalin in the historical reality of the time. The book is primarily oriented toward the criticisms levelled by Kruschev, but Trotsky comes up pretty often as well (being a prominent detractor of Stalin).

It doesn't really define "Trotskyism" in an explicit way, but it does illustrate many differences that Trotsky and Stalin had which you may find helpful.