On Federation starships and space stations, the master systems display (abbreviated MSD), also known as the master situation monitor, or master situational display, is a large, wall-mounted computer display, sometimes in engineering or the bridge. The display usually featured a large cutaway diagram of the vessel, and was used to provide a detailed overview of the ship's status.
The USS Enterprise-D had its master systems display in engineering, along with a master systems display table, while the starships USS Voyager, USS Defiant, USS Hathaway, USS Equinox, USS Enterprise-B, and USS Enterprise-E all featured their displays on the bridge. (Star Trek Generations; Star Trek: The Next Generation, TNG: "Peak Performance"; DS9: "The Search, Part I", etc.; Star Trek: Voyager, VOY: "Equinox", "Equinox, Part II")
Originally, Michael Okuda intended the "Master systems display" and the "Master situation monitor" to be two different objects. According to the Star Trek: The Next Generation Technical Manual (pp. 47-48) and the Star Trek Encyclopedia (3rd ed., pp. 291-292), the "Master systems display" was the information console (affectionately dubbed the "pool table") seen in main engineering on board the USS Enterprise-D, USS Enterprise-E, and in the back of the bridge on the Enterprise NX-01. The "Master situation monitor" was meant to refer to the wall mounted back-lit (cut-away) graphics of the ships in question. Neither of the two designations were initially seen on-screen in their entirety, though there were "Master situation" bridge work stations on both the USS Excelsior and USS Enterprise-A in Star Trek VI: The Undiscovered Country (and, in Excelsior's case, in "Flashback" as well). None of the wall-mounted graphics had a caption "Master systems display" until Doug Drexler updated the cut-away graphic for the Defiant-class. Drexler gave it the caption "master systems display", which is legible in "Shattered Mirror" and "One Little Ship". These were the only such graphics actually carrying the designation, as other MSDs were usually labeled with the ship's name, registry, ship class, random numbering or any combination thereof.
The first MSD was created for the sets of the Enterprise-D. According to the Encyclopedia (3rd ed., p. 291), it included a number of in-jokes, including "the official USS Enterprise duck, the ship's mouse, a Porsche, a DC-3 airplane, the Nomad space probe, and the hamster on a treadmill that was alleged to be the true source of power for the ship's warp engines."