Either:
- A region in west-central Florida including the metropolitan area of Tampa and St. Petersburg as well as lots of other smaller ones. Its borders are vague: Tampabaylive.com considers the area to include Pinellas County, Pasco County, Polk County, Citrus County, Manatee County, Sarasota County, and Hillsborough County; the Tampa Bay Library Consortium includes libraries in these as well as DeSoto, Hardee, Highlands, Hernando, and Okeechobee counties. (Of all these, only Hillsborough, Manatee, and Pinellas counties actually have coastline on the bay.)
- The actual body of water, an arm of the Gulf of Mexico about halfway down the west side of the Florida peninsula. At high tide, it covers 398 square miles, with four major rivers (the Hillsborough, the Alafia, the Manatee and the Little Manatee Rivers) flowing into it as well as numerous smaller rivers and creeks. It averages only 12 feet deep and has had to be dredged to make shipping channels. (On the other hand, the shallowness has made it relatively easy to make bridges across it -- the north-south Sunshine Skyway bridge across the entrance to the bay from the gulf, and the east/west Gandy Bridge, Howard Frankland Bridge, and Courtney Campbell Causeway which all cross the north end of the bay between Hillsborough and Pinellas Counties.)
It's Florida's largest open-water estuary and contains huge amounts of wildlife despite having about 4 billion gallons of hazardous materials passing through it each year from the surrounding human settlements. Until about 1979, the bay was in extremely bad shape with frequent algae blooms and fish dying off, but improvements in sewage treatment and increased environmental awareness have helped the water quality improve and the amount of thriving wildlife increase a lot in the past two decades. However, oil spills and unintended chemical runoff from cities still cause problems.
Sources:
http://www.tbep.org
http://www.tampabaylive.com
http://www.tblc.org