The glycemic index (GI) measures how quickly
carbohydrates enter your blood stream. Carbohydrates that quickly metabolize release glucose into your bloodstream quickly, and so have a high glycemic index, while carbohydrates that break down slowly have a low GI. A lower glycemic index indicates slower rates of digestion and absorption of carbohydrates and therefore sugar.
Dr. David J. Jenkins and colleagues at the University of Toronto invented the Glycemic Index in order to determine the foods that were most appropriate for diabetics. A lower glycemic response usually means a lower insulin demand and so can improve long-term blood sugar control.
To be scientific about this, GI is defined as the area under a two hour blood glucose response curve following the ingestion of 50 of carbohydate. Values range from 0 (not response) to 100 (that from ingesting pure sugar).
A related value is
Glycemic Load.