rogue, n. 1. in general usage, a dishonest or worthless person; connotatively, a charming, rascally ne'er-do-well; c. 1560.
2. in game systems such a AD&D and Rolemaster, a subclass of thief with superior fighting skills.
3. syn. with thief. [GJ1]
Addendum: All thieves are Rogues now since 3rd Edition, but not all Rogues are thieves. Keep this in mind.