California (ˌkælɨˈfɔrnjə) is a state located on the West Coast of the United States. It is the most populous U.S. state, home to 1 out of 8 Americans, and is the third largest state by area (after Alaska and Texas). It is home to the nation's second and fifth largest census statistical areas (Greater Los Angeles area and San Francisco Bay Area, respectively), and eight of the nation's 50 most populated cities (Los Angeles, San Diego, San Jose, San Francisco, Fresno, Sacramento, Long Beach, and Oakland). The capital city is Sacramento.

California is bordered by Oregon to the north, Nevada to the east, and Arizona to the southeast. California is also bordered by Baja California, a state of Mexico, to the south. California's diverse geography ranges from the Pacific Coast in the west, to the Sierra Nevada in the east – from the Redwood–Douglas-fir forests of the northwest, to the Mojave Desert areas in the southeast. The center of the state is dominated by the Central Valley, a major agricultural area. California contains both the highest and lowest points in the contiguous United States (Mount Whitney and Death Valley), and has the 3rd longest coastline of all states (after Alaska and Florida). Earthquakes are a common occurrence due to the state's location along the Pacific Ring of Fire: about 37,000 are recorded annually.

The name California once referred to a large area of North America claimed by Spain that included much of the modern-day Southwestern United States and the Baja California peninsula. Beginning in the late 18th century, the area known as Alta California, comprising the California territory north of the Baja Peninsula, was colonized by the Spanish Empire as part of New Spain. In 1821, Alta California became a part of Mexico following its successful war for independence. Shortly after the beginning of the Mexican-American War in 1846, a group of American settlers in Sonoma declared an independent California Republic in Alta California. Though its existence was short-lived, its flag became the precursor for California's current state flag. American victory in the war led to the Treaty of Guadalupe Hidalgo, in which Mexico ceded Alta California to the United States. Western areas of Alta California became the state of California, which was admitted as the 31st state on September 9, 1850.

The California Gold Rush beginning in 1848 led to dramatic social and demographic change, with large scale immigration from the U.S. and abroad and an accompanying economic boom. Key developments in the early 20th century included the emergence of Los Angeles as the center of the American entertainment industry, and the growth of a large, state-wide tourism sector. The late 20th century saw the development of the technology and information sectors, punctuated by the growth of Silicon Valley (part of the San Francisco Bay Area). California's prosperous agricultural industry also emerged; at least half of the fresh fruit produced in the United States are now cultivated in California, and the state also leads in the production of vegetables. Other important contributors to its economy include aerospace, education, and manufacturing. If California were a country, as of 2012 it would have the 9th largest economy in the world and it would be the 34th most populous nation.