Medical Definition of PolydipsiaPolydipsia: Constant, excessive drinking as a result of thirst. Polydipsia occurs in untreated or poorly controlled diabetes mellitus. Source: MedTerms™ Medical DictionaryLast Editorial Review: 5/13/2016Medical Dictionary Definitions A - ZSearch Medical Dictionary- The Basics of a Healthy Diabetes Diet- How Well Are You Living With Diabetes?- What Can You Eat With Diabetes?