breastbone

[brest-bohn]
noun
the sternum.

Origin:
before 1000; Middle English brust-bon, Old English brēostbān. See breast, bone1

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Collins
World English Dictionary
breastbone (ˈbrɛstˌbəʊn)
 
n
the nontechnical name for sternum

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Etymonline
Word Origin & History

breastbone
"sternum," O.E. breostban, from breast + bone.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

breastbone breast·bone (brěst'bōn')
n.
See sternum.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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American Heritage
Science Dictionary
breastbone   (brěst'bōn')  Pronunciation Key 
See sternum.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Encyclopedia Britannica
Encyclopedia

breastbone

in the anatomy of tetrapods (four-limbed vertebrates), elongated bone in the centre of the chest that articulates with and provides support for the clavicles (collarbones) of the shoulder girdle and for the ribs. Its origin in evolution is unclear. A sternum appears in certain salamanders; it is present in most other tetrapods but lacking in legless lizards, snakes, and turtles (in which the shell provides needed support). In birds an enlarged keel develops, to which flight muscles are attached; the sternum of the bat is also keeled as an adaptation for flight

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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Example sentences
It stretches from the bottom of the breastbone to the belly button, and
  increases with muscle straining.
Or the cancer may have spread to lymph nodes behind the breastbone.
Persistent pain from the breastbone cut is unlikely but possible.
The breastbone is attached to the collarbone and the first seven ribs.
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