In physics, the period of an oscillation or wave (T) is the time elapsed between two equivalent points on the wave. The concept appears in mathematics as well as physics and other areas of knowledge.

It is the minimum period that separates two instants in which the system is in exactly the same state: same positions, same speeds, same amplitudes. Thus the period of oscillation of a wave is the time it takes to complete a wavelength. In brief terms, it is the time it takes for a wave cycle to start over. For example, in a wave "Wave (physics)"), the period is the time elapsed between two successive crests or troughs. The period (T) is the inverse of the frequency (f):.

Since the period is always inverse to the frequency, the wavelength is also related to the period, through the propagation speed formula. In this case the propagation speed will be the ratio between the wavelength and the period.

In physics, periodic motion is always bounded motion, that is, it is confined to a finite region of space from which the particles never leave.

For sufficiently small motion it can be represented by a quasi-harmonic motion of the form:

The term is the phase, being the initial phase, it is the angular frequency, giving the approximate relationship:.

Depending on the degree of approximation of how close the energy is to the minimum, for energies just above the minimum the motion is very close to the harmonic motion given by:.

A period of a real function f is a number such that for all t it holds that:.

Note that in general there is an infinite number of T values ​​that satisfy the previous condition, in fact the set of periods of a function forms an additive subgroup of . For example, it has as a set of periods a , the multiples of 2yuya.

A sum of periodic functions is not necessarily periodic, as seen in the following figure with the function cos t + cos(√2·t):.

To be so, the quotient of the periods must be rational; when this last condition is not met, the resulting function is said to be quasiperiodic.