Webster's Revised Unabridged Dictionary · 1913

Tertian

Tertian , adjective

[Latin tertianus, from tertius the third. See Tierce.]

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

Tertian , noun

[Latin tertiana (sc. febris): compare Old French tertiane.]

1.
(Medicine) A disease, especially an intermittent fever, which returns every third day, reckoning inclusively, or in which the intermission lasts one day.
2.
A liquid measure formerly used for wine, equal to seventy imperial, or eighty-four wine, gallons, being one third of a tun.