Jayhawker and red leg are terms that came to prominence in Kansas Territory during the Bleeding Kansas period of the 1850s; they were adopted by militant bands affiliated with the free-state cause during the American Civil War. These gangs were guerrillas who often clashed with pro-slavery groups from Missouri, known at the time in Kansas Territory as "Border Ruffians" or "Bushwhackers". After the Civil War, the word "Jayhawker" became synonymous with the people of Kansas, or anybody born in Kansas. Today a modified version of the term, Jayhawk, is used as a nickname for a native-born Kansan.

The Territory of Kansas was an organized incorporated territory of the United States that existed from May 30, 1854, until January 29, 1861, when the eastern portion of the territory was admitted to the Union as the free state of Kansas.