porpoise -
A small cetacean of the family Phocoenidae, related to whales and dolphins.
porpoise -
Any small dolphin.
Verb
porpoise -
Said of an aircraft: to make a series of plunges when taking off or landing.
Wikipedia
Porpoises (also called mereswine) are small cetaceans of the family Phocoenidae; they are related to whales and dolphins. They are distinct from dolphins, although the word "porpoise" has been used to refer to any small dolphin, especially by sailors and fishermen. The most obvious visible difference between the two groups is that porpoises have flattened, spade-shaped teeth distinct from the conical teeth of dolphins, and shorter beaks.
OmegaWiki Dictionary
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porpoise A short-snouted genus of the dolphin family, 1.2 to 2.5 metres (4 to 8 feet) long, gregarious in habits, yielding an oil and leather.