Regions of the brain and spinal cord that are made up primarily of cell bodies and dendrites of nerve cells rather than of myelinated axons.
Compare: white matter
Source:
PAC, 2016, 88, 713. 'Glossary of terms used in developmental and reproductive toxicology (IUPAC Recommendations 2016)' on page 762 (https://doi.org/10.1515/pac-2015-1202)