San Francisco (initials SF; / , -/, Spanish for 'Saint Francis'; Spanish: ), officially the City and County of San Francisco, is the cultural, commercial, and financial center of Northern California. The consolidated city-county covers an area of about 46.89 square miles (121.4 km2), mostly at the north end of the San Francisco Peninsula in the San Francisco Bay Area. It is the fourth-most populous city in California and the 13th-most populous in the United States, with a 2017 census-estimated population of 884,363. San Francisco is also the fifth most densely populated U.S. county, behind only four New York City boroughs. As of 2016, San Francisco County was the 7th highest-income county in the United States, with a per capita personal income of $110,418.
San Francisco was founded on June 29, 1776, when colonists from Spain established Presidio of San Francisco at the Golden Gate and Mission San Francisco de Asís a few miles away, all named for St. Francis of Assisi. The California Gold Rush of 1849 brought rapid growth, making it the largest city on the West Coast at the time. San Francisco became a consolidated city-county in 1856. After three-quarters of the city was destroyed by the 1906 earthquake and fire, San Francisco was quickly rebuilt, hosting the Panama-Pacific International Exposition nine years later. In World War II, San Francisco was a major port of embarkation for service members shipping out to the Pacific Theater. It then became the birthplace of the United Nations in 1945. After the war, the confluence of returning servicemen, significant immigration, liberalizing attitudes, along with the rise of the "hippie" counterculture, the Sexual Revolution, the Peace Movement growing from opposition to United States involvement in the Vietnam War, and other factors led to the Summer of Love and the gay rights movement, cementing San Francisco as a center of liberal activism in the United States. Politically, the city votes strongly along liberal Democratic Party lines.
A popular tourist destination, San Francisco is known for its cool summers, fog, steep rolling hills, eclectic mix of architecture, and landmarks, including the Golden Gate Bridge, cable cars, the former Alcatraz Federal Penitentiary, Fisherman's Wharf, and its Chinatown district. San Francisco is also the headquarters of five major banking institutions and various other companies such as Levi Strauss & Co., Gap Inc., Fitbit, Salesforce.com, Dropbox, Reddit, Square, Inc., Dolby, Airbnb, Weebly, Pacific Gas and Electric Company, Yelp, Pinterest, Twitter, Uber, Lyft, Mozilla, Wikimedia Foundation, Craigslist, and Weather Underground. It is home to a number of educational and cultural institutions, such as the University of San Francisco (USF), University of California, San Francisco (UCSF), San Francisco State University (SFSU), the De Young Museum, the San Francisco Museum of Modern Art, and the California Academy of Sciences.
San Francisco has several nicknames, including "The City by the Bay", "Golden Gate City", "Frisco", "SF", "San Fran", and "Fog City", as well as older ones like "The City that Knows How", "Baghdad by the Bay", "The Paris of the West", or, as locals call it, "The City". As of 2017, San Francisco is ranked high on world liveability rankings.