Where does the name "B-Tension" come from?
The B-screw is for "Body screw", since it affects the body angle of the derailleur.

B-Tension is Shimano's term for B-screw -- as Sheldon points out, on Campagnolo, the screw behaves the opposite of Shimano. 

See Sheldon Brown and Park Tool for more details. Also, see the answer here for more details on what it does.