Balaenoptera borealis, commonly known as the Sei whale, is a species of baleen whale found in the oceans around the world. The Sei whale is the third-largest rorqual after the blue and fin whales and is characterized by its long, thin body and narrow, pointed head. Some of the synonyms for Balaenoptera borealis include the Sardine whale, Rudolph's whale, and Pollack whale. The Sei whale is an important species for commercial whaling, and its populations were severely depleted due to overhunting. However, conservation efforts have resulted in the recovery of some populations, and the species is now considered vulnerable by the IUCN.