The sei whale (/ˈseɪ/ or /ˈsaɪ/), Balaenoptera borealis, is a baleen whale, the third-largest rorqual after the blue whale and the fin whale. It inhabits most oceans and adjoining seas, and prefers deep offshore waters. It avoids polar and tropical waters and semi-enclosed bodies of water. The sei whale migrates annually from cool and subpolar waters in summer to winter in temperate and subtropical waters.