Anti-aliasing
Softening the staircase along diagonal edges by blending in intermediate colours — the cure for pixels being square.
A diagonal line drawn on a grid becomes a staircase. Anti-aliasing fixes this by giving pixels along the edge an intermediate colour reflecting how much of the pixel the line actually covers. The eye reads the result as a smoother edge.
The methods differ in cost. MSAA computes several sample points per pixel and is accurate but expensive. FXAA works on the finished image looking for edges — cheap but blurry. TAA blends in previous frames, giving excellent still images but ghosting in motion. The argument about TAA's softness is one of the more durable quarrels in PC gaming.