Source : Webster's Revised Unabridged Dictionary (1913)

Superposition \Su`per*po*si"tion\, n. [Cf. F. superposition. See
   {Super-}, and {Position}.]
   The act of superposing, or the state of being superposed; as,
   the superposition of rocks; the superposition of one plane
   figure on another, in geometry.

Source : WordNet®

superposition
     n 1: (geology) the deposition of one geological stratum on
          another
     2: (geology) the principle that in a series of stratified
        sedimentary rocks the lowest stratum is the oldest [syn: {principle
        of superposition}, {superposition principle}]
     3: (geometry) the placement of one object ideally in the
        position of another one in order to show that the two
        coincide
     4: the placement of one thing on top of another