Period:

In a periodic ac wave, the mathematical function of amplitude versus time repeats accurately and indefinitely; the similar pattern persists countless times. The period is the length of time among one repetition of the pattern, or one wave cycle, and later. This is demonstrated in figure shown below for a simple ac wave.

Figure: A sine wave. The period is the length of time needed for one cycle to be completed.

The period of a wave, in hypothesis, can be anyplace from a very small fraction of a second to many centuries. Few electromagnetic (EM) fields have periods measured in quadrillionths of a second or lesser. The charged particles held captive by the magnetic field of the Sun reverse their direction over periods measured in years. Period, whenever measured in seconds, is denoted by T.