The fundamental principle of physics that the total energy of an isolated system is constant despite internal changes WordNet 3.0 & Open English WordNet

In physics and chemistry, the law of conservation of energy states that the total energy of an isolated system remains constant; it is said to be conserved over time. Wikipedia

The first law of thermodynamics is a version of the law of conservation of energy, adapted for thermodynamic processes, distinguishing three kinds of transfer of energy, as heat, as thermodynamic work, and as energy associated with matter transfer, and relating them to a function of a body's state, called internal energy. Wikipedia

Principle that the total energy of an isolated system remains constant over time Wikipedia Disambiguation

Law of physics and chemistry Wikidata

Statement of conservation of energy as it applies specifically to a thermodynamic system or process Wikidata

The law stating that the total amount of energy in any isolated system remains constant, and cannot be created or destroyed, although it may change forms. Wiktionary

Total energy remains constant. Wiktionary (translation)