Lewis & Short

tertĭārĭus, a, um, adj. [tergeo], containing a third part.

  1. I. Adj.: stannum, that contains one part of white and two of black lead, Plin. 34, 17, 48, § 160.
  2. II. Subst.: tertĭārĭum, ii, n., a third part, a third, Cato, R. R. 95, 1; Vitr. 4, 7, 5.