Entry 3 senses · 2 variants Webster, 1913 Tertian · Ter·tian · IPA a. n. 01 a. Occurring every third day; as, a tertian fever. 1. Occurring every third day; as, a tertian fever.(Med.) 02 n. A disease, especially an intermittent fever, which returns every third day, reckoning inclusively, or in which the intermission lasts one day. 1. A disease, especially an intermittent fever, which returns every third day, reckoning inclusively, or in which the intermission lasts one day.(Med.) 2. A liquid measure formerly used for wine, equal to seventy imperial, or eighty-four wine, gallons, being one third of a tun.