Fill rate
How many pixels or texels a GPU can write per second — for a long time the hardest bottleneck in real-time graphics.
The term covers two measures: pixel fill rate (how many pixels the ROP units can write to the framebuffer) and texel fill rate (how many texture samples are taken). The number is roughly unit count times clock speed, quoted in gigapixels per second.
In the 1990s, during the 3dfx Voodoo era, fill rate decided almost everything — higher resolution and more overdraw consume it linearly. Today's cards more often move the bottleneck to memory bandwidth and shading, but fill rate still caps how much transparency and post-processing a scene can bear.