Frames Per Second
From Diablo Wiki
Frames Per Second, or FPS refers to how many times a computer manages to update its animations on the screen per second. It's a universal benchmark to gauge how well a game runs on certain hardware.
Around 24 FPS is bearable in a game, feeling pretty smooth. Above is good, and below is very bad.
Under 1 FPS, or if it's generally slow, it's common to call the experience a "slideshow".
FPS is not to be confused with the other FPS, the First Person Shooter - a game genre.