perpetual motion

from The American Heritage® Dictionary of the English Language, 5th Edition.

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

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The motion of some hypothetical device that continues forever with no obvious input of energy in violation of the laws of thermodynamics.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun motion that continues indefinitely without any external source of energy; impossible in practice because of friction