a rorqual (Balaenoptera musculus) with a blue-gray back: the largest animal that has ever lived, reaching a length of over 30 m (100 ft) and weighing up to 136,000 kg (c. 150 tons)
See blue whale in American Heritage Dictionary 4
A very large whalebone whale (Balaenoptera musculus) having a bluish-gray back, yellow underparts, and several ventral throat grooves. It is considered the largest living animal, sometimes reaching a length of 30.5 meters (100 feet). Also called sulphur-bottom.