This is an article about the physical phenomenon of wave interference. See also the legal concept of humanitarian interference
Interference is the superposition (overlapping) of two or more waves resulting in a new wave pattern. As most commonly used, the term usually refers to the interference of waves of the same or nearly the same frequency.
The principle of superposition of waves states that the resultant displacement at a point is equal to the sum of the displacements of different waves at that point. If a crest of a wave meets a crest of another wave at the same point then the crests interefere constructively and the resultant wave amplitude is greater. If a crest of a wave meets a trough then they interfere destructively, and the overall amplitude is decreased.
Interference is involved in Thomas Young's double-slit experiment where coherent light (light which is in the same phase, and has the same frequency and wavelength) interferes to produce an interference pattern. More generally, this form of interference can occur whenever a wave can propagate from a source to a destination by two or more paths of different length. Two or more sources can only be used to produce interference when there is a fixed phase relation between them, but in this case the interference generated is the same as with a single source; see Huygens' principle. When a single source interferes with itself, the principle of conservation of energy dictates that the energy "missing" from the darkened regions of an interference pattern where destructive interference has taken place will be found in the brightened portions where constructive interference has taken place.
Light beams that can produce interference patterns are called "coherent," and have all of their photons' phases aligned with each other. Lasers are an example of a coherent light source. Light beams which cannot produce interference patterns are called "incoherent." Most ordinary incandescent light sources (including the Sun) are incoherent.
In a signal transmission system, "interference" refers to either extraneous power which tends to interfere with the reception of the desired signals or to the disturbance of the signals which results.