Periodicity is the quality or character of being periodic; the tendency to recur at intervals. If a body returns to a particular place at a regular interval or passes a particular place at regular interval then that is called periodic motion. For example, the earth rotates around the sun and comes to a particular place after 365 days, a simple pendulum starting from one end goes to the other end and comes back to the original end or, an oscillator generates wave and it is observed that at particular points (here point A or point B) the amplitude of the wave at regular interval repeats (Figure). All these are the examples of periodic motion.
The time period may be with respect to time, again with respect to position.
The earth comes back to a place rotating around the sun in 365 days. This time is called time period. So, the time period of rotation of the earth around the sun is 365 days. Similarly, for a simple pendulum time taken in between starting from a point and returning to that point is called its time period. In the figure, a time taken by a particle to travel from point A to point E is the time period λ.
