> Since memory was tight originally, the language design for MUMPS valued very terse code. Thus, every MUMPS command or function name could be abbreviated from one to three letters in length, e.g. Quit (exit program) as Q, $P = $Piece function, R = Read command, $TR = $Translate function. Spaces and end-of-line markers are significant in MUMPS because line scope promoted the same terse language design. Thus, a single line of program code could express, with few characters, an idea for which other programming languages could require 5 to 10 times as many characters. Abbreviation was a common feature of languages designed in this period (e.g., FOCAL-69, early BASICs such as Tiny BASIC, etc.). An unfortunate side effect of this, coupled with the early need to write minimalist code, was that MUMPS programmers routinely did not comment code and used extensive abbreviations. This meant that even an expert MUMPS programmer could not just skim through a page of code to see its function but would have to analyze it line by line.
https://en.wikipedia.org/wiki/MUMPS#Overview
I'll add that Wikipedia's take here is a little optimistic: perhaps it was true when MUMPS was invented that it could use one line to accomplish what would require "5 to 10 times as many characters" in other languages, but MUMPS's language constructs are by and large about as low level as assembly with a couple exceptions, so this is definitely no longer the case if we're comparing it to modern high level languages.
In addition to Epic, the US Department of Veterans Affairs wrote their homegrown EMR in MUMPS. Their development group is here: https://groups.google.com/forum/#!forum/hardhats