The Collaborative International Dictionary of English v.0.48:

Stereogram \Ste"re*o*gram\, n. [Stereo- + -gram.]
   A diagram or picture which represents objects in such a way
   as to give the impression of relief or solidity; also, a
   stereograph.
   [1913 Webster]

The Free On-line Dictionary of Computing (30 December 2018):

stereogram

   A two-dimensional image which, when viewed correctly appears
   three-dimensional.  E.g. SIRDS or SIRTS.

   (ftp://katz.anu.edu.au/pub/stereograms).

   (1994-11-03)