Gamma-Aminobutyric acid (γ-Aminobutyric acid) (also called GABA for short) is the chief inhibitory neurotransmitter in the mammalian central nervous system. It plays the principal role in reducing neuronal excitability throughout the nervous system. In humans, GABA is also directly responsible for the regulation of muscle tone.

Although in chemical terms it is an amino acid(as it has both a primary amine and a carboxylic acid functional group), GABA is rarely referred to as such in the scientific or medical community. By convention the term "amino acid", when used without a qualifier, refers specifically to an alpha amino acid. GABA is not an alpha amino acid, meaning the amino group is not attached to the alpha carbon so it is not incorporated into proteins.