| In U.S. history, the doctrine that the people in a territory were the ones who could choose whether that territory would allow slavery. (Some felt this decision should be made as soon as there were a substantial number of settlers in the area; others that it could be put off until that territory wanted to become a state.) This was meant as a sort of compromise between people in the North who were generally anti-slavery in new states, and people in the South who were for it, because both sides felt that more states on their side would allow their side of the issue to prevail in Congressional voting.
Stephen A. Douglas probably coined the actual phrase, but the idea had been proposed by others before. This became a big issue in the 1840s and 1850s because so much land had been acquired for the U.S. in the previous half-century and was getting a large number of settlers for the first time. The idea was incorporated in the Compromise of 1850 and four years later was an important feature of the Kansas-Nebraska Act, even though Kansas was in an area that the Missouri Compromise of 1820 had said was to be free. The Kansas-Nebraska Act was written on the assumption that Nebraska would become a free state, Kansas a slave state, and the two would cancel one another out, but Nebraskans and Missourians crossed into Kansas and often physically fought over the issue. The controversy in Kansas probably helped the American Civil War start more quickly. | 
