In physics, a conservation law states that a particular measurable property of an isolated physical system does not change as the system evolves.
One particularly important physical result concerning conservation laws is Noether's Theorem, which states that there is a one-to-one correspondence between conservation laws and differentiable symmetries of physical systems. For example, the conservation of energy follows from the time-invariance of physical systems, and the fact that physical systems behave the same regardless of how they are oriented in space gives rise to the conservation of angular momentum.
A partial listing of conservation laws that are said to be exact laws, or more precisely have never been shown to be violated:
There are also approximate conservation laws. These are approximately true in particular situations, such as low speeds, short time scales, or certain interactions.