What is Missouri Compromise?

Legal Definition
The Missouri Compromise was attained through legislation passed by the 16th Congress of the United States on April 2, 1820. The measures provided for the admission of the District of Maine as a free state and the Missouri territory without restriction on slavery. In addition, it outlawed slavery north of the 36°30′ parallel within the Louisiana Purchase lands, thereby committing the largest remaining portion of the territory to free-soil. South of the parallel no slavery restrictions were imposed. President James Monroe signed the legislation on April 6, 1820.

The compromise bills served to quell the furious sectional debates that had first erupted during the final session of the 15th Congress. On February 3, 1819, Representative James Tallmadge, Jr., a Jeffersonian Republican from New York State, had submitted two amendments to Missouri's request for statehood. The first proposed to federally prohibit further slave migration into Missouri; the second would require all slave offspring, born after statehood, freed at 25 years of age. At issue among southern legislators was the encroachment by their northern free state colleagues in what they considered a purely sectional concern: slave labor.

Northern critics including Federalists and Republicans, objected to the expansion of slavery into the Louisiana Purchase territory on the Constitutional inequalities of the three-fifths rule, which conferred Southern representation in the federal government, derived from a states’ slave population. Nonetheless, the more populous North held a firm numerical advantage in the House. Jeffersonian Republicans in the North ardently maintained that a strict interpretation of the Constitution required that Congress act to limit the spread of slavery on egalitarian grounds.

The slave-holding states were acutely aware that maintaining a balance in the number of free-to-slave states was necessary to ensure political equilibrium in the US Senate. With the Senate evenly split at the opening of the debates, both sections possessing 11 states, the admission of Missouri would give the South a two-seat advantage in the upper house and diminish the Northern lower house majority. The South sought to enlist Missouri to maintain Southern political preeminence and ensure security of their institutions.

The Missouri question in the 15th Congress ended in stalemate on March 4, 1819, the House sustaining its northern antislavery position, and the Senate blocking a slavery restricted statehood. Antislavery agitation grew in the North in the aftermath of the debates, leading to widespread opposition to slavery in Missouri. As the 16th Congress assembled in December 1819, the two houses remained thoroughly polarized over slavery in the Louisiana Purchase territories.

When the free-soil District of Maine offered its petition for statehood, the Senate quickly linked the Maine and Missouri bills, making Maine admission a condition for Missouri entering the Union with slavery unrestricted. Senator Jesse B. Thomas of Illinois added a compromise proviso, excluding slavery from all remaining lands of the Louisiana Purchase north of the 36 30’ parallel. The combined measures passed the Senate, only to be voted down in the House by those Northern representatives who held out for a free Missouri. Speaker of the House of Representatives Henry Clay of Kentucky, in a desperate bid to break the deadlock, divided the Senate bills. Clay and his pro-compromise allies succeeded in pressuring half the anti-restrictionist House Southerners to submit to the passage of the Thomas proviso, while maneuvering a number of restrictionist House northerners to acquiesce in supporting Missouri as a slave state. This was the Missouri Compromise.

The legislation extracted by the compromisers served to effect a "brokered truce" or "armistice" rather than a genuine compromise. The crux of the Compromise was that it circumvented the deepening disaffection among Jeffersonian Republicans.

The Missouri crisis would spur the formation of two powerful political organizations – the Democratic and Whig Parties – both committed to preserving the federal Union by means of sectional compromise and the suppression of the explosive proslavery and antislavery arguments that had surfaced over Missouri statehood. The repeal of the Missouri Compromise in the Kansas-Nebraska Act of 1854 would hasten the growth of a mass antislavery coalition – the Republican Party –whose precepts of which were first formulated by Jeffersonian Republican restrictionists during the Missouri crisis.
-- Wikipedia
Legal Definition
The provisions in the act admitting Missouri to the Union in 1820, prohibiting slavery north of lat. 36° 30' N., except in Missouri.
-- Ballentine's Law Dictionary