Atm is an acronym for ass-to-mouth established in the 1600's by Analucious the third. A fairly popular sexual act involving a man's penis, a woman's anus, and a woman's mouth. This act was perfected by Analucious' sister and his second cousin.
A scientific measure of pressure equivalent to about 14.6959488 pounds per square inch. Abbreviation for "atmosphere." Its is a measurement of how much pressure the atmosphere exerts on an object at sea level (where the pressure would be 1 atm).
There is 2.5818 atm of pressure on that balloon, causing it to shrink from its original size when the pressure was 2 atm.