The Collaborative International Dictionary of English v.0.48:

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

The Collaborative International Dictionary of English v.0.48:

Tertian \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.
      [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]

WordNet (r) 3.0 (2006):

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