a. & n. [ From L. tertius third, the tertial feathers being feathers of the third row. See Tierce. ] (Zool.) Same as Tertiary. [ 1913 Webster ]

a. [ L. tertianus, from tertius the third. See Tierce. ] (Med.) Occurring every third day; as, a tertian fever. [ 1913 Webster ]

n. [ L. tertiana (sc. febris): cf. OF. tertiane. ] 1. (Med.) A disease, especially an intermittent fever, which returns every third day, reckoning inclusively, or in which the intermission lasts one day. [ 1913 Webster ]

2. A liquid measure formerly used for wine, equal to seventy imperial, or eighty-four wine, gallons, being one third of a tun. [ 1913 Webster ]

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. [ 1913 Webster ]

2. (Geol.) The Tertiary era, period, or formation. [ 1913 Webster ]

3. (Zool.) One of the quill feathers which are borne upon the basal joint of the wing of a bird. See Illust. of Bird. [ 1913 Webster ]

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. [ 1913 Webster ]

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. [ 1913 Webster ]

3. (Geol.) Later than, or subsequent to, the Secondary. [ 1913 Webster ]

4. (Zool.) Growing on the innermost joint of a bird's wing; tertial; -- said of quills. [ 1913 Webster ]

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. [ 1913 Webster ]

v. t. [ L. tertiatus, p. p. of tertiare to do for the third time, fr. tertius the third. ] 1. To do or perform for the third time. [ Obs. & R. ] Johnson. [ 1913 Webster ]

2. (Gun.) To examine, as the thickness of the metal at the muzzle of a gun; or, in general, to examine the thickness of, as ordnance, in order to ascertain its strength. [ 1913 Webster ]

‖ [ L. ] A third somewhat; something mediating, or regarded as being, between two diverse or incompatible substances, natures, or positions. [ Webster 1913 Suppl. ]