gamma-aminobutyric acid

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An amino acid, C₄H₉NO₂, that is not found in proteins, but occurs in the central nervous system and is associated with the transmission of nerve impulses.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun biochemistry, physiology An amino acid found in the nervous systems of vertebrates and acting as an inhibitory neurotransmitter. Abbreviated as GABA.