The only thing that doesn't cause confusion is the name used in Linneus' classification, Gadhus Morhua. This name indicates the white cod commonly fished in the waters of the Atlantic, between Canada and Iceland, and as far east as Norway and the Baltic Sea. The general term cod is often incorrectly used for other similar fish, such as the hake.
But the real trouble starts when you come to talk about the "preservable" version: it's often hard to know what's what between stockfish and salt cod. The first to conserve fish in salt were the Basques, and then the Portuguese, who would cover it in salt for a few days before putting it to dry. There it is called "bacalhau", and in Italian it is similarly known as "baccalà". The word appears to come from the Spanish, "bacalao", which in turn would seem to derive from the Latin "baculus", or stick. So "baccalà" is none other than a stick-fish, because of its hardness, of course.
In Portugal, fresh cod is also known as "bacalhau", and is generically called “bacalhau fresco”. Stockfish, on the other hand, is air-dried, as has always been the tradition in Scandinavia. The word comes from the Norwegian stokk fisk, where stokk is the name of the poles used to hang out the fish to dry. In Italian, too, it is called stoccafisso, except in the Veneto region, where it is known as "bacalà". So ultimately, only one thing is certain: whatever you call it, it's always a wonderfully tasty dish.
Photo courtesy Gyurgyak.blogspot.com