noun Anatomy.
the coccyx.

1540–50; tail1 + bone

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American Heritage
Medical Dictionary

tailbone tail·bone (tāl'bōn')
See coccyx.

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
tailbone   (tāl'bōn')  Pronunciation Key 
See coccyx.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Encyclopedia Britannica


curved, semiflexible lower end of the backbone (vertebral column) in apes and humans, representing a vestigial tail. It is composed of three to five successively smaller caudal (coccygeal) vertebrae. The first is a relatively well-defined vertebra and connects with the sacrum; the last is represented by a small nodule of bone. The spinal cord ends above the coccyx. In early adulthood the coccygeal vertebrae fuse with each other; in later life the coccyx may fuse with the sacrum. A corresponding structure in other vertebrates, such as birds, may also be called a coccyx.

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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Example sentences
Osborn based this supposition on what he thought was a change in tailbone
  anatomy about halfway down the organ's length.
He was experiencing some dizziness following the play, but those symptoms
  cleared up and his tailbone injury lasted longer.
Helps reduce the lower back pain, specially in the tailbone.
She continues to complain of a burning pain at her tailbone and a heavy feeling
  in her legs.
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