The Collaborative International Dictionary of English v.0.48:

Dentil \Den"til\, n. [LL. dentillus, for L. denticulus. Cf.
   Dentelli, Denticle, Dentile.] (Arch.)
   A small square block or projection in cornices, a number of
   which are ranged in an ornamental band; -- used particularly
   in the Ionic, Corinthian, and Composite orders.
   [1913 Webster]