Sei is the Norwegian word for pollock, a fish that appears seasonally along the Norwegian coast. The whales showed up on the same schedule, following the same plankton blooms rather than the fish themselves, and whalers named them after the more familiar arrival. The name entered English intact and is pronounced roughly as say.
The diet is mostly copepods, small crustaceans a few millimetres long, plus krill and some schooling fish. To take them, the sei whale often skims: it swims forward with the mouth partly open, letting water pass continuously through unusually fine baleen, in the manner of a right whale rather than the gulping lunges its close relatives use. It can also lunge feed.
Sei whales are also fast. Speeds around 50 kilometres per hour have been reported over short bursts, among the highest for any large whale, though sustained cruising is far slower.