Diabetes: Usually refers to diabetes mellitus or, less often, to diabetes insipidus. Diabetes mellitus and diabetes insipidus share the name "diabetes" because they are both conditions characterized by excessive urination (polyuria).
The word "diabetes" is from the Greek word meaning "a siphon" because people with diabetes "passed water like a siphon."
When "diabetes" is used alone, it refers to diabetes mellitus. The two main types of diabetes mellitus -- insulin-requiring type 1 diabetes and adult-onset type 2 diabetes -- are distinct and different diseases in themselves.