Definition of "porpoise" [por•poise]

• Any of several gregarious toothed whales of the genus Phocaena and related genera, of oceanic waters, characteristically having a blunt snout and a triangular dorsal fin. Also called sea hog. (noun)
• Any of several related aquatic mammals, such as the dolphin. (noun)

American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

• "Many people are of opinion that the porpoise is a variety of the dolphin."
• "A porpoise is a vertical oscillation where you are just a step behind the aircraft and can't physically keep up with the machine; each control movement only serves to exaggerate the problem."
• "The dolphin is so uniformly miscalled porpoise, on the west coast and everywhere else, that the creature will soon come to think that it really is a porpoise."