The Collaborative International Dictionary
Abaculi
Abaculus \A*bac"u*lus\ ([.a]b*[a^]k"[-u]*l[u^]s), n.; pl.
Abaculi (-l[imac]). [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.
Wiktionary
abaculi
n. (en-irregular plural of: abaculus)