The principle of superposition
Interference phenomena occur for any media where waves exist. Electromagnetic waves are such an example. The principle of superposition is a simple statement of the fact that if a sine-wave-like form is a solution to Maxwell's equations, that the sum of any two such waves is also a solution. Therefore, if we consider light emited from two sources, the net electric field is the sum of the electric field of each source. This is not too deep, nonetheless is gets a "principle" asscociated with it.