<p><strong>Noun:</strong></p><ul><li>Any of several small, gregarious cetacean mammals of the genus Phocoena, usually blackish above and paler beneath, and having a blunt, rounded snout, especially the common porpoise, P. phocoena, of both the North Atlantic Ocean and the Pacific Ocean.</li><li>Any of several small cetacean mammals, as the common dolphin, Delphinus delphis.</li></ul><p><strong>Verb:</strong></p><ul><li>(Used without object) To leap clear of the water after striking a wave (of a speeding motorboat).</li><li>To appear above the surface of the water (of a torpedo).</li><li>To move forward with a rising and falling motion in the manner of a porpoise: The car has a tendency to porpoise when overloaded.</li></ul>