Tucson (pronounced /ˈtusɑn/, Spanish: Tucsón) is a city and the seat of Pima County, Arizona, United States, located 118 miles (188 km) southeast of Phoenix. As of July 1, 2005 a census estimate put the city's population at 521,605, and the metropolitan population at 931,210. By 2008 the city population is expected to exceed 610,000, while the metropolitan population is expected to exceed 1 million by spring 2007. In 2005 Tucson ranked as the 32nd-largest city and 52nd-largest metropolitan area in the U.S. It is the largest city in southern Arizona, and the second largest in the state after Phoenix, Arizona.

Major incorporated suburbs of Tucson include Oro Valley and Marana northwest of the city, South Tucson (surrounded by Tucson), and Sahuarita south of the city. Communities in the vicinity of Tucson (some within or overlapping the city limits, and some incorporated in their own right) include Casas Adobes, Catalina, Catalina Foothills, Flowing Wells, Green Valley, Tanque Verde, and Vail.

The name Tucson originates via Spanish from the O'odham, Cuk Ṣon (pronounced [ʧʊk ʂɔn]; roughly, "chuk shon"), meaning "Black Base," a reference to the mostly volcanic mountains on the west side of the city. The most notable of these mountains is Sentinel Peak, better known as "A Mountain" because it sports a large letter A in honor of the nearby University of Arizona, situated in west central Tucson. Tucson is sometimes referred to as "The Old Pueblo."