Motion
Periodic motion is any motion that is repeated at regular intervals. One period is the time in seconds that it takes for one event to occur, and is represented by the symbol T. Frequency is the rate at which these events occur, and is represented by the symbol f, and the unit Hertz (1/sec). Period and frequency are inversely related:
Periodic motion comes up in physics a lot, especially when talking about light or fluid dynamics— it typically manifests in the form of waves. Wavelength is the peak-to-peak distance of a certain wave.