DEFINITION of 'Silicon Valley'. A part of the San Francisco Bay Area that is known for the many technology companies that have either started in the area or that have offices there.Major companies located in Silicon Valley include Google, Apple, Facebook and Yahoo.Silicon Valley is one of the wealthiest areas of the United States. In May 2012, the New York Times reported that 14% of households in Santa Clara county, located in Silicon Valley, and in San Mateo County, earn over $200,000 per year. part of the San Francisco Bay Area that is known for the many technology companies that have either started in the area or that have offices there.