Tertiary \Ter"ti*a*ry\, n.; pl. {Tertiaries}. 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}. --Addis & Arnold. 2. (Geol.) The Tertiary era, period, or formation. 3. (Zo["o]l.) One of the quill feathers which are borne upon the basal joint of the wing of a bird. See Illust. of {Bird}.