Ter″ti‐a‐ry (?), a. [L. tertiarius containing a third part, fr. tertius third: cf. F. tertiaire. See Tierce.] 1. Being of the third formation, order, or rank; third; as, a tertiary use of a word. Trench.
2. (Chem.) Possessing some quality in the third degree; having been subjected to the substitution of three atoms or radicals; as, a tertiary alcohol, amine, or salt. Cf. Primary, and Secondary.
3. (Geol.) Later than, or subsequent to, the Secondary.
4. (Zoöl.) Growing on the innermost joint of a bird's wing; tertial; — said of quills.
Tertiary age. (Geol.) See under Age, 8. — Tertiary color, a color produced by the mixture of two secondaries. “The so-called tertiary colors are citrine, russet, and olive.” Fairholt. — Tertiary period. (Geol.) (a) The first period of the age of mammals, or of the Cenozoic era. (b) The rock formation of that period; — called also Tertiary formation. See the Chart of Geology. — Tertiary syphilis(Med.), the third and last stage of syphilis, in which it invades the bones and internal organs.