Definition of "perpetual motion" []

• The hypothetical continuous operation of an isolated mechanical device or other closed system without a sustaining energy source. (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.

• "Medieval and Renaissance perpetual motion ma - chines, indeed machines in general, were characterized by an exuberant delight in elaboration: gear trains were added merely for the sake of complexity as well as in the hope of hitting on a more subtle science of mechanics that would transcend mere earthbound me - chanics."
• "The quest for perpetual motion was abandoned, for the logical implication of the concept of efficiency is that restoration, or recov - ery, of the initial situation is the utmost that can be expected even of an ideal engine."
• "The last aspect of medieval and Renaissance technics that we shall mention was the proliferation of attempts to devise a perpetual motion machine."