Bay Area is a reference to the San Fransisco Bay area of California.
The Bay Area is a populous region surrounding the San Francisco, San Pablo, and Suisun Bay estuaries in Northern California.
The Bay Area is defined by the Association of Bay Area Governments to include the nine counties that border the estuaries: Alameda, Contra Costa, Marin, Napa, San Mateo, Santa Clara, Solano, Sonoma, and San Francisco.
Among locals, the nine-county Bay Area can be further divided into five sub-regions: the East Bay, North Bay, South Bay, Peninsula, and the city of San Francisco. Although geographically located on the tip of the San Francisco Peninsula, the city of San Francisco is not considered part of the “Peninsula” subregion, but as a separate entity.
The “East Bay” is the densest region of the Bay Area outside of San Francisco and includes cities and towns in Alameda and Contra Costa counties, centered on Oakland.
The “Peninsula” subregion includes the cities and towns on the San Francisco Peninsula, excluding the titular city of San Francisco.
The “South Bay” includes all of the rest of the cities in Santa Clara county, centered on San Jose, the largest city in Northern California. It is roughly synonymous with Silicon Valley due to its high concentration of tech companies, although the industry also has a significant presence in the rest of the Bay Area.
The “North Bay” includes Marin, Sonoma, Napa, and Solano counties, and is the largest and least populated subregion.