monosodium glutamate

n.
  1. The monosodium salt of the natural amino acid L-glutamine (C5H8NNaO4), used as a food additive to enhance flavor; abbreviated MSG. It is usually marketed as the monhydrate (C5H8NNaO4.H2O). It is commmonly used in Chinese-American restaurants, and has been identified as a main cause of Chinese restaurant syndrome.