The area surrounding the San Francisco Bay. Includes North Bay, East Bay, South Bay and the Peninsula.
Do noth confuse with wannabe Bay Areas like Memphis.
99.999999999999999999% of the time it's refering to the place in the NorCal.
John: I'm from the Bay Area.
Dan: Cool, how's the weather?
Dan: Oh, so you're definitely not talking about the REAL Bay Area in California.
John: Uhhh I was talking about Memphis.
Dan: Dude, ur hella crazy. Get with it!