Abbreviation for monosodium glutamate.
It's a chemical food enhancer, extracted from seaweed and other vegetable matter. It's used in prepared seasoning blends, processed foods and a lot of Chinese food. It resembles salt and has been used by the Chinese for centuries to deepen and bring out natural food flavors. Many places now advertise their products as "No MSG" or "MSG Free" because it sometimes triggers migraine attacks in susceptible individuals.