Source : Webster's Revised Unabridged Dictionary (1913)

Polydipsia \Pol`y*dip"si*a\, n. [NL., fr. Gr. poly`s much + ?
   thirst.] (Med.)
   Excessive and constant thirst occasioned by disease.

Source : WordNet®

polydipsia
     n : excessive thirst (as in cases of diabetes or kidney
         dysfunction)