Renegade witchers are witchers who have either betrayed their brethren or been expelled from a witcher school. This usually occurred for a crime such as the killing of an innocent man or other acts equally unworthy of a witcher. There are no references to renegade witchers in Sapkowski's original works. The term appears in the film and television series and in The Witcher 3: Wild Hunt journal, in reference to Letho.
The Season of Storms informs that at least some of the Cat School members are banned from entering Kaer Morhen and don't have a roof over their heads, which could imply that Cats are technically renegades in the Wolves' eyes.