A Conservation Law is the expression of constancy within a system during change. Specifically, given an isolated system, a quantity is conserved if it remains constant despite whatever changes occur to the system. Some examples include:
Conservation of momentum: The principle that the linear momentum of a system (in Newtonian mechanics, mass times velocity) remains the same as long as no external force acts.
Conservation of angular momentum: The principle that the angular momentum of a system (the momentum of rotation about a point) remains the same as long as no external torque acts.
Conservation of energy: The principle that the total energy of a closed system never changes, that energy is only converted from one form to another. This principle must be enlarged under special relativity to include mass-energy.
Conservation of matter: The principle that matter is neither created nor destroyed. This principle is only approximately true, since it is violated by special relativity.
Conservation of charge: The principle that electrical charge is neither created nor destroyed in particle interactions. The total electrical charge in an isolated system remains unchanged regardless of interactions or particle transformations.