Why is it called Diabetes Mellitus?

Diabetes comes from Greek, and it means a siphon. Aretus the Cappadocian, a Greek physician during the second century A.D., named the condition diabainein. He described patients who were passing too much water (polyuria) - like a siphon. The word became "diabetes" from the English adoption of the Medieval Latin diabetes.

In 1675 Thomas Willis added mellitus to the term, although it is commonly referred to simply as diabetes. Mel in Latin means honey; the urine and blood of people with diabetes has excess glucose, and glucose is sweet like honey. Diabetes mellitus could literally mean "siphoning off sweet water".

In ancient China people observed that ants would be attracted to some people's urine, because it was sweet. The term "Sweet Urine Disease" was coined.
There are three main types of diabetes:

Diabetes Type 1 - You produce no insulin at all.
Diabetes Type 2 - You don't produce enough insulin, or your insulin is not working properly.
Gestational Diabetes - You develop diabetes just during your pregnancy.

(World Health Organization)

Diabetes Types 1 & 2 are chronic medical conditions - this means that they are persistent and perpetual. Gestational Diabetes usually resolves itself after the birth of the child.
Treatment is effective and important

All types of diabetes are treatable, but Type 1 and Type 2 diabetes last a lifetime; there is no known cure. The patient receives regular insulin, which became medically available in 1921. The treatment for a patient with Type 1 is mainly injected insulin, plus some dietary and exercise adherence.

Patients with Type 2 are usually treated with tablets, exercise and a special diet, but sometimes insulin injections are also required.

If diabetes is not adequately controlled the patient has a significantly higher risk of developing complications, such as hypoglycemia, ketoacidosis, and nonketotic hypersosmolar coma. Longer term complications could be cardiovascular disease, retinal damage, chronic kidney failure, nerve damage, poor healing of wounds, gangrene on the feet which may lead to amputation, and erectile dysfunction.