Glucose: The simple sugar that is the chief source of energy. Glucose is found in the blood and is the main sugar that the body manufactures. The body makes glucose from all three elements of food'protein, fats, and carbohydrates'but the largest amount of glucose derives from carbohydrates. Glucose serves as the major source of energy for living cells. However, cells cannot use glucose without the help of insulin. Also known as dextrose.