In physics, coherence is an ideal property of waves that enables stationary (i.e. temporally and spatially constant) interference. It contains in fact several distinct concepts, which are limit cases that never occur in reality but allows to understand the physics of waves and has become in particular a very important concept in quantum physics. 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 at this point that interference is nothing more than the addition, in a mathematical sense, of wave functions. In quantum physics, a single wave can interfere with itself, but this is due to its quantum behavior and is still an addition of two waves (see Young's slits experiment). This implies that constructive or destructive interferences are limit cases, and that waves can always interfere, even if the result of the addition is complicated or not remarkable.
When interfering, two waves can add together to create a wave of greater amplitude than either one (constructive interference) or subtract from each other to create a wave of lesser amplitude than either one (destructive interference), depending on their relative phase. Two waves are said to be coherent if they have a constant relative phase. The degree of coherence is measured by the interference visibility, a measure of how perfectly the waves can cancel due to destructive interference.
Temporal and Spatial Coherence can be exhibited by Michelson–Morley experiment and Young's slits experiment respectively. Once the infringes are obtained in Michelson–Morley experiment, if now one of the mirror is moved away gradually then the time for the beam to travel increases and the infringes become dull and finally are lost, showing Temporal Coherence. Similarly if in Young's double slit experiment if the space between the two slits is increased, the coherence dies gradually and finally the infringes disappear , showing Spatial Coherence.