Definition of "keystone" []

• Architecture The central wedge-shaped stone of an arch that locks its parts together. Also called headstone. (noun)
• The central supporting element of a whole. (noun)

American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

• "Someone has to hold the arch together while the keystone is put in place, or they are thrown together at a single time improbably falling neatly into place."
• "Wolves are called keystone predators, because their presence or absence has a radical effect on a whole complex eco-system."
• "At its simplest, a keystone is a device for harvesting and storing power."