A maelstrom is a very powerful whirlpool; a large, swirling body of water. A free vortex, it has considerable downdraft. The power of tidal whirlpools tends to be exaggerated by laymen. There are few stories of large ships ever being sucked into a maelstrom, although smaller craft are in danger and tsunami or sinkhole-generated maelstroms may even threaten larger craft. Tales like those by Paul the Deacon, Edgar Allan Poe, and Jules Verne are entirely fictional.

One of the earliest uses in English of the Scandinavian word (malström or malstrøm) was by Edgar Allan Poe in his story "A Descent into the Maelström" (1841). In turn, the Nordic word is derived from the Dutch maelstrom, modern spelling maalstroom, from malen (to grind) and stroom (stream), to form the meaning grinding current or literally "mill-stream", in the sense of milling (grinding) grain.