Meaning of triple-expansion

tri•ple-ex•pan•sion

Pronunciation: (trip"ul-ik-span'shun), [key]
— adj.
  1. noting a power source, esp. a steam engine, using the same fluid at three successive stages of expansion to do work in three or more cylinders.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.