Definition of "backbone" []

• The vertebrate spine or spinal column. (noun)
• Something, such as the keel of a ship, that resembles a backbone. (noun)
• A main support or major sustaining factor: the backbone of a thesis. (noun)
• Geology A ridge forming the principal axis of a mountain. (noun)
• Geology The principal mountain ridge, range, or system of a region. (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.

• "The yellow streak up his backbone is the width of the M25."
• "LOTHIAN: As President Obama signed executive orders that he says levels the playing field between labor unions and employers, he reached out to what he calls the backbone of the country -- middle class families, people White House spokesman Robert Gibbs suggested the Bush administration ignored."
• "He said the problem with recruiting and retaining the general internal medicine specialists, which he called the backbone of rural hospitals, plus support staff, has put pressure on family physicians to deal with sick patients beyond their scope."