Coherent light

Coherent light are light waves that are "in phase" with one another.

For example, two waves are coherent if the crests of one wave are aligned with the crests of the other and the troughs of one wave are aligned with the troughs of the other. Otherwise, these lightwaves are considered incoherent.

[ Two coherent waves ]

Light produced by lasers are coherent light. Light from light bulbs or the sun, however, are incoherent light.

See also:
Holograms
Lasers
Interference
Light Bulbs and Fires
TV and Florescent Light
The Sun