Mm is defined as a way to say yes or maybe to a question.(interjection)
An example of mm is saying you don't know if you'll be going to an event.
Mm is defined as millimeter.(abbreviation)
An example of mm is how someone would shorten the phrase "100 millimeters;" 100 mm.
See mm in Webster's New World College Dictionary
See mm in American Heritage Dictionary 4
Learn more about mm