Medical Definition of CRYPTOMNESIA. : the appearance in consciousness of memory images which are not recognized as such but which appear as original creations.â crypÂ·tomÂ·neÂ·sic \-ËnÄ-zik, -sik\ adjective. the appearance in consciousness of memory images which are not recognized as such but which appear as original creations. â crypÂ·tomÂ·neÂ·sic \-ËnÄ-zik, -sik\ adjective.