The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

superimpose
    v 1: place on top of; "can you superimpose the two images?"
         [syn: superimpose, superpose, lay over]