1. [adj]  of or relating to a tonal system based on major thirds; "a tertian tonal system"
2. [adj]  relating to symptoms (especially malarial fever) that appear every other day; "tertian fever"

TERTIAN is a 7 letter word that starts with T.

1. \Ter"tian\, a. [L. tertianus, from tertius the third.
   See {Tierce}.] (Med.)
   Occurring every third day; as, a tertian fever.
   
   

2. \Ter"tian\, 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.
   
   2. A liquid measure formerly used for wine, equal to seventy
      imperial, or eighty-four wine, gallons, being one third of
      a tun.