Short for mushroom button, most often used as a triggering device for an E-Stop.
Photo from this thread: http://www.controlbooth.com/forums/general-advice/13405-totally-random-where-do-you-buy-these.html.
Photo from this thread: http://www.controlbooth.com/forums/general-advice/13405-totally-random-where-do-you-buy-these.html.