Private Branch eXchange. It's a telephone switch, like the ones the phone company has at the central office only on a smaller scale. It lets a business (or home,if you're insane) do things like share 4 phone lines among 16 extensions, allowing 4 people to call the outside world at once and the other 12 to call within the PBX if necessary. These things are a pain in the butt to program, because the people who sell you the PBX also want to sell you their programming services.See Merlin Legend.