The Collaborative International Dictionary of English v.0.48:

Abaciscus \Ab`a*cis"cus\ ([a^]b`[.a]*s[i^]s"k[u^]s), n. [Gr.
   'abaki`skos, dim of 'a`bax. See Abacus.] (Arch.)
   One of the tiles or squares of a tessellated pavement; an
   abaculus.
   [1913 Webster]