Definition of "porpoise"

porpoise

/ˈpɔːpəs/

noun
plural porpoises
1. A small cetacean of the family Phocoenidae, related to dolphins and whales.
  Quotations
  They captured 25 porpoises and set them free back into the Yangtze after raising them for a month in Puqi County, Hubei.
  1991, Kaiya Zhou, Zhang Xingduan, translated by Luo Changyan, Baiji: The Yangtze River Dolphin and Other Endangered Animals of China‎, page 76
2. (Canada, US, imprecisely) Any small dolphin.
verb
third-person singular simple present porpoises, present participle porpoising, simple past and past participle porpoised
1. (intransitive) Said of an air-breathing aquatic animal such as a porpoise or penguin: To repeatedly jump out of the water to take a breath and dive back in a continuous motion.
  Quotations
  Urging them to join me, I raced in circles through the surging water, chased my tail for the children, blew spouts of foam through the sunfilled spray, porpoised to and fro across the river in shallow leaps that stitched the air and water into a table-lace of foam.
  1979, J.G. Ballard, The Unlimited Dream Company, chapter 15
2. (intransitive) Said of an aircraft: to make a series of plunges when taking off or landing; or of a watercraft: to successively plunge up and down in the water.