Interference is the overlapping of two or more waves resulting in a new wave pattern.
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.
See also interferometer.