California is a state in the Western United States, lying on the American Pacific Coast. It borders Oregon to the north, Nevada and Arizona to the east, and an international border with the Mexican state of Baja California to the south. With nearly 39 million residents across an area of 163,696 square miles (423,970 km²), it is the most populous U.S. state, the third-largest by area, and most populated subnational entity in North America.

Prior to European colonization, California was one of the most culturally and linguistically diverse areas in pre-Columbian North America. European exploration in the 16th and 17th centuries led to the colonization by the Spanish Empire. The area became a part of Mexico in 1821, following its successful war for independence, but was ceded to the United States in 1848 after the Mexican–American War. The California gold rush started in 1848 and led to social and demographic changes, including depopulation of Indigenous peoples in the California genocide. The western portion of Alta California was then organized and admitted as the 31st state in 1850, as a free state, following the Compromise of 1850.

The Greater Los Angeles and San Francisco Bay areas are the nation's second- and fifth-most populous urban regions, with 19 million and 10 million residents respectively. Los Angeles is the state's most populous city and the nation's second-most; California's capital is Sacramento. The state's diverse geography ranges from the Pacific Coast and metropolitan areas in the west to the Sierra Nevada mountains in the east, and from the redwood and Douglas fir forests in the northwest to the Mojave Desert in the southeast. Two-thirds of the nation's earthquake risk lies in California. The Central Valley, a fertile agricultural area, dominates the state's center. The large size of the state results in climates that vary from moist temperate rainforest in the north to arid desert in the interior, as well as snowy alpine in the mountains. Droughts and wildfires are an ongoing issue.

California's economy is the largest of any U.S. state, with a $4.0 trillion gross state product as of 2024. It is the largest sub-national economy in the world. California's agricultural industry has the highest output of any U.S. state, and is led by its dairy, almonds, and grapes. With the busiest port in the country (Los Angeles), California plays a pivotal role in the global supply chain, hauling in about 40% of goods imported to the US. Notable contributions to popular culture, ranging from entertainment, sports, music, and fashion, have their origins in California. California is the home of Hollywood, the oldest and one of the largest film industries in the world, profoundly influencing global entertainment. The San Francisco Bay and the Greater Los Angeles areas are seen as the centers of the global technology and U.S. film industries, respectively.