Source : WordNet®

principle of superposition
     n 1: (geology) the principle that in a series of stratified
          sedimentary rocks the lowest stratum is the oldest [syn:
           {superposition principle}, {superposition}]
     2: the displacement of any point due to the superposition of
        wave systems is equal to the sum of the displacements of
        the individual waves at that point; "the principle of
        superposition is the basis of the wave theory of light"
        [syn: {Huygens' principle of superposition}]