abaculus
noun
abac·u·lus
ə-ˈba-kyə-ləs
plural abaculi
ə-ˈba-kyə-ˌlī,
-ˌlē
: a tile used in mosaic : tessera
Word History
Etymology
misreading of Latin ab oculīs "out of view," as if diminutive of abacus abacus
First Known Use
1844, in the meaning defined above