Source : Webster's Revised Unabridged Dictionary (1913)

Superimpose \Su`per*im*pose"\, v. t. [imp. & p. p.
   {Superimposed}; p. pr. & vb. n. {Superimposing}.]
   To lay or impose on something else; as, a stratum of earth
   superimposed on another stratum. -- {Su`per*im`po*si"tion},
   n.

Source : WordNet®

superimpose
     v : place on top of; "can you superimpose the two images?" [syn:
          {superpose}, {lay over}]