I have no idea what ADM stands for off the top of my head but hunting
around for factoids
ftp://ftp.whtech.com/datasheets%2520and%2520manuals/Hardware/ADM3A.pdf is a
manual for one and commonly calls the system an Interactive Display
Terminal but does call the monitor part a "Display Monitor".
http://www.old-computers.com/history/detail.asp?n=32&t=3 admits they aren't
sure but gives a few educated guesses: "The ADM-3A was one of the first
affordable serial display terminals manufactered by Lear-Siegler, Inc of
Anaheim California. Why ADM? Nobody knows, maybe *American Dream Machine*or
*Awful Dumb Monitor* or *Advanced Display Module* or, more seriously, *Anaheim
Division, Manufacturing*..."
You'd think they'd spell it out somewhere in an early manual though.