Pea-sized conical mass of tissue behind the third ventricle of the brain, secreting the hormone-like substance melatonin in some mammals.
Source:
PAC, 2016, 88, 713. 'Glossary of terms used in developmental and reproductive toxicology (IUPAC Recommendations 2016)' on page 792 (https://doi.org/10.1515/pac-2015-1202)