any of several small, gregarious cetaceans of the genus Phocoena, usually blackish above and paler beneath, and having a blunt, rounded snout, esp. the common porpoise, P. phocoena, of both the North Atlantic and Pacific.

any of several other small cetaceans, as the common dolphin, Delphinus delphis.

(of a speeding motorboat) to leap clear of the water after striking a wave.

(of a torpedo) to appear above the surface of the water.

to move forward with a rising and falling motion in the manner of a porpoise

The car has a tendency to porpoise when overloaded.

noun

cetacean , dolphin , mammal , whale