superposition

from WordNet (r) 3.0 (2006)

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},
       {superposition}]
    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

from The Collaborative International Dictionary of English v.0.48

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.
   [1913 Webster]