When two or more identical waves (same wavelength, amplitude and frequency) travel in the same medium at the same time, they interfere each other. Such a mixing up of waves is called interference.

When the two waves are in phase (the crest of one meets the crest of the other or the trough of one meets the trough of the other) they reinforce each other. In such cases the interference is said to be constructive. When the interfering waves are out of phase (the crest of one coincides with the trough of the other) they tend to cancel each other and the interference is said to be destructive.