blue whale

WordReference Random House Unabridged Dictionary of American English © 2025

blue′ whale′, 

1. Mammals a migratory baleen whale, Balaenoptera musculus, mostly of oceans and seas in the Southern Hemisphere, growing to a length of 100 ft. (30.5 m) 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. Also called sulfur-bottom. 

• 1850–55

Collins Concise English Dictionary © HarperCollins Publishers::

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 Balaenopteridae