2 Answers

having four units or components; "quadruple rhythm has four beats per measure"; "quadruplex wire".

More specifically, it's most common etymological meaning (that is, the meaning in most common usage) is from TV broadcasting and refers to the 2-inch magnetic recording tape/mechanism ... "The name 'quadruplex' refers to the use of four magnetic record/reproduce heads mounted on a headwheel spinning transversely (width-wise) across the tape at a rate of 14,400 rpm for NTSC 525 lines/30fps-standard quad decks, and at 15,000 rpm for those using the PAL 625 lines/25fps video standard. This method was called quadrature scanning, as opposed to the helical scan transport used by later videotape formats." (source: Wikipedia -- http://en.wikipedia.org/wiki/Quadraplex) In this usage, quadruplex is governed by TV broadcasting standards set by patents, ISO and other standards bodies as well industry norms. For the disambiguous meaning, see answer 1.