porpoise Definition

any of a family (Phocoenidae) of small gregarious toothed whales; especially : a blunt-snouted usually dark gray whale (Phocoena phocoena) of the North Atlantic and North Pacific that typically ranges from 5 to 6 feet (1.5 to 1.8 meters) in length

dolphin —not used technically

to leap or plunge like a porpoise

to rise and fall repeatedly

a black-and-white porpoise (Phocoenoides dalli) of temperate and arctic waters of the rim of the North Pacific Ocean —called also Dall porpoise