Full-screen Anti-aliasing. A feature of some new graphics chips and/or software programs designed to smooth "jaggies," FSAA is a feature on the 3DFX, Voodoo5, ATI Radeon 256, nVIDIA GeForce 256 and other advanced graphics chips.
Full Screen Anti Aliasing, what is done by videocard to remove the jagged edges in lines. The resulting image is much smoother on the eyes, however FSAA has usually a 50% performance reduction when enabled.