noun U.S. History.
A settlement of a dispute between slave and free states, contained in several laws passed during 1820 and 1821. Northern legislators had tried to prohibit slavery in Missouri, which was then applying for statehood. The Missouri Compromise admitted Missouri as a slave state and Maine as a free state, and prohibited slavery in territory that later became Kansas and Nebraska. In 1857, in the Dred Scott decision, the Supreme Court declared the compromise unconstitutional.