GABA

GABA is "gamma"-Aminobutyric acid (γ-Aminobutyric acid). It is a neurotransmitter in the central nervous system of mammals. It is an "inhibiting" neurotransmitter. Normally, when a neuron receives an impulse, it will make the signal stronger. In the case where there is an "inhibiting" neurotransmitter, the cell will no longer get the impulse, and the signal as a whole will be weakened. 
GABA regulates how much neurons in the central nervous system will be stimulated in humans, and other mammals. It plays a role in regulating neuronal excitability throughout the nervous system. In humans, GABA is also directly responsible for the regulation of muscle tone. In insect species GABA acts only on excitatory nerve receptors.
Even though chemically it is an amino acid, GABA is rarely referred to as such in the scientific or medical communities. The term "amino acid," used without a qualifier, refers to the alpha amino acids, which GABA is not. GABA is also not incorporated into proteins.