Interference is the superposition and resulting algebraic sum of two waves in a vibratory physical phenomenon. Interference occurs in telecommunications, in optics, in acoustics, and in general when wave-type energy is propagated.
A wave phenomenon due to the addition of vibrations that are of the same wavelength and coherent (i.e. having a constant phase difference).
Constructive interference is when the amplitudes of the interfering waves add up at the point considered; destructive interference is when they subtract.
This wave phenomenon especially affects light and other types of electromagnetic waves, and also sound. It is responsible for the dark fringes in the interference pattern of two point sources (the double slit experiment), or the fading of direct radio waves or those reflected from certain layers of the atmosphere (cf. fading).
By extension, the term interference denotes any parasites due to the superposition of any kind of waves.