New and abnormal formation of tissue as a consequence of growth by cell proliferation, which may continue after the initial stimulus that initiated the proliferation has ceased, and which may develop into a tumor.
Source: PAC, 2016, 88, 713. 'Glossary of terms used in developmental and reproductive toxicology (IUPAC Recommendations 2016)' on page 783 (https://doi.org/10.1515/pac-2015-1202)