Commonly used abbreviation for monophonic, meaning only one channel or "track" of sound. Contrast with stereo and stereophonic.
Also
As mentioned above, an abbreviation for the highly infectious but non-lethal disease mononucleosis, also known as "the kissing disease." It is a myth that one can only catch mono by kissing, as it is also transmitted by sharing utensils (things like cups, forks, and the food they touch) with an infected person. It will indeed knock its victim flat on his or her ass, sometimes for months at a time. It will, too, make its victims wish they were dead.