a state in the southwestern US, on the border with Mexico; population 6,500,180 (est. 2008); capital, Phoenix; statehood, Feb. 14, 1912 (48). Part of New Spain until 1821, it was organized as a US territory in 1863 from lands ceded by the Treaty of Guadalupe Hidalgo in 1848 and the Gadsden Purchase in 1853.