From AMS Glossary
The superposition of two or more waves resulting in an amplitude of the composite wave not necessarily the algebraic sum of the amplitudes of each of its components.
The simplest example is the superposition of two one-dimensional scalar waves with equal amplitude A:
The amplitude B of the sum of these waves lies between 0 (destructive interference) and 2A (constructive interference) depending on the phase difference Δφ = φ2 - φ1:
Destructive and constructive interference are the two extremes of interference not, as is often implied, the only two possibilities. Interference requires coherence of the waves, by which is meant a definite and fixed phase difference between them.