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bellybutton

or bel·ly but·ton

[bel-ee-buht-n]
noun Informal.
  1. the navel.

Origin of bellybutton

An Americanism dating back to 1875–80; belly + button
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018
British Dictionary definitions for belly button

bellybutton

noun
  1. an informal name for the navel
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
Word Origin and History for belly button
n.

"navel," 1877, colloquial, from belly (n.) + button (n.). Also bellybutton, belly-button.

Online Etymology Dictionary, © 2010 Douglas Harper
belly button in Medicine

bellybutton

[bĕl′ē-bŭt′n]
n.
  1. navel
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.