Frames Per Second
The resolution of video in the time dimension. More frames per second means smoother motion. Low FPS means motion will appear "choppy".
24-30 FPS is generally considered good - it is the level at which motion will appear smooth to most people. 60 FPS video is also common and may appear smoother.
Some devices can display 90 or higher FPS, most commonly for games. However, for displays, this is usually expressed in Hertz instead of FPS, although the concepts are similar (Hertz is a measure of something per second).
Phones with a slow-motion video feature typically capture at higher FPS rates (120 and higher) and then play back that video at a normal viewing FPS (30-60) in order to create the smooth slow-motion effect.