The term is an acronym for Private Branch eXchange. A PBX, sometimes known as a telephone switch or phone switching device, is a system that connects office telephone extensions with the public network.
Our company uses an IP PBX as a communication server.
A polymer-bonded explosive, also called PBX or plastic-bonded explosive, is an explosive material in which explosive powder is bound together in a matrix using small quantities (typically 5–10% by weight) of a synthetic polymer ("plastic"). Note that despite the word "plastic", polymer-bonded explosives are not hand malleable after curing, and hence are not a form of plastic explosive.
PBXs are normally used for explosive materials which are not easily meltable to cast or which are otherwise hard to form.
JACK: So, we plant some PBX, jump ship, then blow it.
THOR: What is PBX?
TEAL'C: An explosive.