For the ancient Greeks, diabetes meant polyuria and polydipsia (too frequent urination causing severe dehydration causing severe thirst causing the diabetic to drink a lot of water). There were two types of diabetes: mellitus, where the urine was sweet, and insipidus, where it was not.
Today, insipidus is not taught to primary care physicians in the US (but it is in the UK), so there was a New York Times story about a woman with insipidus who was told her excessive drinking of water was a psychological problem for 30 years before she found a specialist who knew about insipidus and how to treat it. Diabetes in the US now means diabetes mellitus, always (since very few US physicians know there is any other kind). Wikipedia says diabetes insipidus is not diabetes at all, and must never be confused with diabetes, since diabetes must always mean mellitus. The new and improved definition of diabetes is a fasting blood sugar of more than 126 mg/dl or 7 mmol/L.
So then diabetes had two types, Juvenile and Adult. Juvenile diabetes occurred in childhood, and required insulin, while Adult diabetes occurred later in life and was not responsive to insulin, but must be treated with other medications. This did not always work out very well when an adult’s islets of Langerhans took early retirement and the physicians insisted it must be treated without insulin.
After that, diabetes had three types, Type I, where the patient had no insulin (which always results in high blood sugar), and Type 2, where the patient had insulin and high blood sugar, and gestational diabetes that only occurred during pregnancy (and did not have a numbered Type).
This has changed again, so now there are 5 types of diabetes: Type 1 has antibodies, and antibodies are the only test for Type 1. Type 2 has no insulin and no antibodies, and is no longer considered Type 1. If your C-Peptide is 0, but you have no antibodies, you are the new and improved Type 2. Type 3 is severe insulin resistant diabetes. Type 4 is mild obesity caused insulin resistant diabetes. And Type 5 is mild age-related insulin resistant diabetes. The new Types 1 and 2 both used to be called Type 1, while Types 3, 4, and 5 all used to be called Type 2, but modern medicine has moved beyond that.
(I don’t see the old gestational diabetes anywhere among the new 5 Types, but then it never did have a numbered Type associated with it. Adding it in, there are 6 Types, of which 5 are numbered and one is not.)