Definition of "cornerstone" []

• A stone at the corner of a building uniting two intersecting walls; a quoin. (noun)
• Such a stone, often inscribed, laid at a ceremony marking the origin of a building. (noun)
• An indispensable and fundamental basis: the cornerstone of an argument. (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.

• "That's what prompted President Roosevelt to launch the Social Security system in 1935, to create what he called the cornerstone of a civilized society."
• "America and its allies gear up for their big fight of the summer-to extend control over the southern province of Kandahar, which they call the cornerstone of the counterinsurgency campaign-the Taliban have been on a killing spree."
• "And that less visibility basically is sort of the right sort of the, what I call the cornerstone of [which drove the] guidance."