>>some of Escher's illusions are 'tessel[l]ations'.
>Sorry, but the word tessellations does not refer to the optical illusion, but to the geometric fact of covering a surface...

and I was referring to the fact that these works of Escher are commonly referred to as tessellations -- he used this technique to create these particular illusions. [see the link] - as I said, this is of no help in the word search, merely a punctilio of interest.