Period

A period is the time it takes for something that repeats to repeat itself. For example, the second hand on a clock has a period of one minute, because it takes it one minute to go all the way around the clock and get back to where it started.

Things that move in a way that they repeat themselves are called "periodic". The hands on a clock are periodic. The voltage of an electrical oscillator is periodic. The Earth is periodic in two different ways: it rotates completely on its axis with a period of 24 hours, and it revolves around the Sun with a period of one year.