conservation of mass

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

• noun A principle in classical physics stating that the total mass of an isolated system is unchanged by interaction of its parts.

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

• noun physics, chemistry A conservation law of classical physics that states that the total mass of a closed system remains constant regardless of the chemical or physical changes that take place within it.

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

• noun a fundamental principle of classical physics that matter cannot be created or destroyed in an isolated system