California, constituent state of the United States of America. It was admitted as the 31st state of the union on September 9, 1850, and by the early 1960s it was the most populous U.S. state. No version of the origin of California’s name has been fully accepted, but there is wide support for the contention that it derived from an early 16th-century Spanish novel, Las sergas de Esplandián (“The Adventures of Esplandián”), that described a paradisiacal island full of gold and precious stones called California. The influence of the Spanish settlers of the 18th and 19th centuries is evident in California’s architecture and place-names. The capital is Sacramento.