This is what a unit means in reference to insulin:
Insulin is measured in units, abbreviated U (international units, previously abbreviated IU).
One unit of insulin is defined as the amount of insulin that will lower the blood glucose of a healthy 2 kg (4.4 lb) rabbit that has fasted for 24 hours to 2.5 mmol/l (45 mg/dl) within 5 hours.
Insulins vary in their concentration. A U100 insulin is 2.5 times more concentrated than A U40 insulin. However, a unit of insulin is a unit of insulin regardless of the concentration.