being of the third formation, order, or rank; third; as, a tertiary use of a word.-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.-later than, or subsequent to, the secondary.-growing on the innermost joint of a bird's wing; tertial; -- said of quills.-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.-the tertiary era, period, or formation.-one of the quill feathers which are borne upon the basal joint of the wing of a bird. see illust. of bird.