Its an acronym for Master Systems Display or Master Situational Display. It is also known as Master Situational Monitor. It is usually a large display in Engineering or on the Bridge that is a line diagram cross section in profile of the ship and gives status information about the starship's systems. It is of interest toTreknology fans because it gives us an idea of the internal layout of the ship. In the case of the Galaxy-
, the MSD contains many jokes like showing a Porsche 911 in the main shuttle bay. A large rubber duck in one of the rooms while a DC-3 is found in another.
Memory Alpha describes it and has graphic images of the MSDs created for the series and movies: http://memory-alpha.org/en/wiki/Master_systems_display