Emergency Stop. A safety switch required on anything mechanized, as a life-safety measure. Sometimes called a "mushroom button" due to its shape. Often there will be more than one, ideally at convenient locations. An E-Stop should, safely, remove all sources of energy to a moving object, and possibly engage brakes or other locking mechanisms. Some sort of manual override, or human intervention/inspection is usually required before an E-Stop condition may be cleared.
http://www.grainger.com/Grainger/items/6HK79?cm_mmc=Google Base-_-Electrical-_-Switches-_-6HK79
http://www.grainger.com/Grainger/items/6HK79?cm_mmc=Google Base-_-Electrical-_-Switches-_-6HK79