Diabetes insipidus is a medical condition caused when vasopressin does not appear in a person's blood. Vasopressin is released from the brain, and travels in the blood to the kidneys. There, it pulls water out of the urine, which reduces the amount of urine stored in the bladder.
If there is no vasopressin in the blood, the kidneys leave the water in the urine, and the bladder holds more liquid. This means a person needs to urinate more often. It also causes thirst. Increased thirst and urination are symptoms of regular diabetes (diabetes mellitus). That's why diabetes insipidus shares the name "diabetes."