What does "polydipsia" mean and what conditions cause it?

Polydipsia means excessive thirst, from poly meaning many or excessive and dipsia meaning thirst. It is a classic symptom of diabetes mellitus where high blood sugar causes osmotic diuresis, meaning the kidneys excrete large volumes of urine pulling water with the glucose, leading to dehydration and compensatory thirst. The three classic signs of diabetes are polydipsia, polyuria meaning excessive urination, and polyphagia meaning excessive hunger.