a. | 1. | Being of the third formation, order, or rank; third; as, a tertiary use of a word. |
| 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. | (Zool.) Growing on the innermost joint of a bird's wing; tertial; - said of quills. |
n. | 1. | (R. C. Ch.) A member of the Third Order in any monastic system; as, the Franciscan tertiaries; the Dominican tertiaries; the Carmelite tertiaries. See Third Order, under Third. |
| 2. | (Geol.) The Tertiary era, period, or formation. |
| 3. | (Zool.) One of the quill feathers which are borne upon the basal joint of the wing of a bird. See Illust. of Bird. |