Non-intentional termination of pregnancy before the embryo or fetus has developed to the stage of independent viability, or in humans before the 20th week of gestation.
Source:
PAC, 2016, 88, 713. 'Glossary of terms used in developmental and reproductive toxicology (IUPAC Recommendations 2016)' on page 715 (https://doi.org/10.1515/pac-2015-1202)