If you're a diabetic, you are a person who suffers from the disease of diabetes. Diabetics tend to have wildly varying blood sugar levels, which can sometimes be dangerous.
A diabetic's pancreas typically doesn't produce enough insulin, the hormone in your body that metabolizes sugars and carbohydrates. The word diabetic is also an adjective that describes someone who has the illness or something associated with it. Your diabetic friend might need to monitor her blood sugar several times a day and follow a special diabetic diet. The word comes from the Greek, in which it means "excessive discharge of urine," one of the illness's symptoms.