The Collaborative International Dictionary of English v.0.48:

monosodium glutamate \monosodium glutamate\ n.
   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.
   [WordNet 1.5]