Bacon is the cured and/or smoked side, belly or back of a pig.
American bacon—called streaky bacon by our British friends—is cut from a side of fatty pig belly. It’s normally sold in slices (“rashers” across the pond). Canadian bacon (English back bacon) is made from the leaner loin, located in the back.
While bacon usually implies “pork,” the word can be used to describe cuts (and composite cuts) from other animals, such as turkey, as well as “veggie bacon” made with soy.