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blue whale

noun
  1. a migratory baleen whale, Balaenoptera musculus, mostly of oceans and seas in the Southern Hemisphere, growing to a length of 100 feet (30.5 meters) and having a furrowed, slate-blue skin mottled with lighter spots, in some seas acquiring a yellowish coating of diatoms on the underside: the largest mammal ever known, it is now endangered.
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Origin of blue whale

First recorded in 1850–55
Also called sulfur-bottom.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

British Dictionary definitions for blue whale

blue whale

noun
  1. the largest mammal: a widely distributed bluish-grey whalebone whale, Sibbaldus (or Balaenoptera) musculus, closely related and similar to the rorquals: family BalaenopteridaeAlso called: sulphur-bottom
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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