**Coherence / Coherent** Coherence is an ideal property of waves that enables stationary interference.

More generally, coherence describes all properties of the correlation between physical quantities of a single wave, or between several waves or wave packets.

One should note that constructive or destructive interferences are limit cases, waves can always interfere, even if the result of the addition is complicated or not remarkable.

The coherence of two waves follows from how well correlated the waves are as quantified by the cross-correlation function. The cross-correlation quantifies the ability to predict the value of the second wave by knowing the value of the first.

Temporal coherence is the measure of the average correlation between the value of a wave and itself delayed by τ, at any pair of times.

Spatial coherence describes the ability for two points in space, x1 and x2, in the extent of a wave to interfere, when averaged over time.

__Source:__ http://en.wikipedia.org/wiki/Coherence_%28physics%29