a rorqual (Balaenoptera borealis) with a light-gray or bluish back, found in all seas
See sei (whale) in American Heritage Dictionary 4
noun
A rorqual (Balaenoptera borealis) that is blue-black above and white below, that grows up to about 55 feet (17 meters) in length, and that is found in all the oceans, with the greatest number living in Antarctic waters.