Two years ago, when I was in the Occupy movement, my comrades and I argued about revolution. Was revolution necessary? What is it? The split that destroyed our movement — as it did the Left during the 1960s — pitted revolutionaries against reformists. The most frustrating part of the debate, however, wasn't ideological. It was linguistic.

Even on the Left, few Americans know what revolution is: the violent overthrow of the ruling classes. In a revolution, everything — beginning with the power structure — changes.

The Tahrir Square encampments that led to the ouster of Egyptian autocrat Hosni Mubarak were a huge influence on Occupy. But we couldn't agree about what they meant. Was Tahrir a "revolution"?