sei whale(n = noun.animal) balaenoptera borealis - similar to but smaller than the finback whale;
is a kind of razorback, rorqual
is a member of balaenoptera, genus balaenoptera

baleen whale, beaked whale, black whale, blue whale, bottle-nosed whale, bottlenose whale, bowhead whale, dwarf sperm whale, fin whale, finback whale, gray whale, greenland whale, grey whale, humpback whale, killer whale, minke whale, piked whale, pilot whale, pygmy sperm whale, right whale, sperm whale, toothed whale, whale, whale louse, whale oil, whale shark, whale sucker, whalebone whale, white whale