conservation of mass

American Heritage® Dictionary of the English Language, Fourth Edition

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

Wiktionary

1. n. 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.

WordNet 3.0

1. n. a fundamental principle of classical physics that matter cannot be created or destroyed in an isolated system