A fundamental principle of classical physics that matter cannot be created or destroyed in an isolated system WordNet 3.0 & Open English WordNet

In physics and chemistry, the law of conservation of mass or principle of mass conservation states that for any system closed to all transfers of matter and energy, the mass of the system must remain constant over time, as the system's mass cannot change, so the quantity can neither be added nor be removed. Wikipedia

Principle that the mass of any closed system must remain constant over time Wikipedia Disambiguation

Scientific law stating that form of matter cannot lose or gain mass Wikidata

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. Wiktionary

Conservation law. Wiktionary (translation)