(1) Interference is an optical effect which can occur when two or more Light beams are superimposed. (2) Unwanted electrical signals or Noise causing Degradation of reception on a communications circuit.
More precisely, for interference to occur, several conditions have to be met:
spatial and temporal overlap of the two light fields
In this case, the resulting total light field does not have an Intensity which equals the sum of the intensities of the superimposed beams. Instead, its complex Amplitude is the sum of the amplitudes of the superimposed beams. This makes a big difference: for example, the amplitudes of two equally intense light beams can have opposite signs, SO that they can cancel each other. On the other hand, with equal signs (phases) of both contributions, the total intensity can be four times that of the single beams. Although this might look different, Energy is conserved in any case. For example, IF two light beams of equal intensity, Frequency and polarization are superimposed on a screen with some angle, one obtains an interference pattern consisting of bright and dark stripes, and the spatially averaged intensity equals the sum of the intensities of the single beams.