Source : Webster's Revised Unabridged Dictionary (1913)

   3. (Geol.) Later than, or subsequent to, the Secondary.

   4. (Zo["o]l.) Growing on the innermost joint of a bird's
      wing; tertial; -- said of quills.

   {Tertiary age}. (Geol.) See under {Age}, 8.

   {Tertiary color}, a color produced by the mixture of two
      secondaries. ``The so-called tertiary colors are citrine,
      russet, and olive.'' --Fairholt.

   {Tertiary period}. (Geol.)
      (a) The first period of the age of mammals, or of the
          Cenozoic era.
      (b) The rock formation of that period; -- called also
          {Tertiary formation}. See the Chart of {Geology}.

   {Tertiary syphilis} (Med.), the third and last stage of
      syphilis, in which it invades the bones and internal
      organs.

Source : WordNet®

tertiary syphilis
     n : the third stage; characterized by involvement of internal
         organs especially the brain and spinal cord as well as
         the heart and liver