(1):

(n.) A liquid measure formerly used for wine, equal to seventy imperial, or eighty-four wine, gallons, being one third of a tun.

(2):

(a.) Occurring every third day; as, a tertian fever.

(3):

(n.) A disease, especially an intermittent fever, which returns every third day, reckoning inclusively, or in which the intermission lasts one day.