abaculus
Webster's Unabridged Dictionary:
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A·bac·u·lus
n. (ȧb*ăk"ū̍*lŬs) pl. Abaculi (-lī).
[L., dim. of abacus.]
(Arch.) A small tile of glass, marble, or other substance, of various colors, used in making ornamental patterns in mosaic pavements. Fairholt.
