Conservation law

A rule stating that a measurable quantity remains constant during a specified physical process. Conservation laws constrain how physical systems can change and connect symmetries to invariant quantities.

Noether's theorem: A theorem linking continuous symmetries of a system's action to conserved quantities. It explains why symmetries generate conservation laws in classical and quantum physics.
Closed system: A system that exchanges no matter with its surroundings, though it may exchange energy. Conservation statements depend on which exchanges across the system boundary are allowed.
Conservation of energy: The principle that total energy remains constant in an isolated system. Energy conservation is the most widely applied instance of the general rule.
Conservation of mass: The classical principle that total mass remains constant in a chemical or material process. Relativity refines this classical rule by relating mass to energy.
Continuity equation: A local equation expressing how a conserved density changes through flux and sources. It translates global conservation into a rule for transport through space.
Isolated system: A system that exchanges neither matter nor energy with its surroundings. Isolation supplies the conditions for familiar global conservation statements.
Conservation of momentum: The principle that total momentum remains constant when a system has no net external force. It predicts outcomes of collisions and constrains the motion of interacting bodies.
Mass–energy equivalence: The relation between mass and energy expressed by the equation E = mc². It shows why mass alone need not remain constant when energy changes.
Symmetry in physics: An invariance of physical laws under a transformation of space, time, or other variables. Invariance under transformations is the structural source of many conservation laws.
Physical quantity: A measurable property represented by a number and, often, a unit. A conservation law applies to a defined measurable quantity, not to an unspecified property.

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