Definition of tertiary

• a. - Being of the third formation, order, or rank; third; as, a tertiary use of a word.
• a. - 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.
• a. - Later than, or subsequent to, the Secondary.
• a. - Growing on the innermost joint of a bird's wing; tertial; -- said of quills.
• n. - 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.
• n. - The Tertiary era, period, or formation.
• n. - One of the quill feathers which are borne upon the basal joint of the wing of a bird. See Illust. of Bird.