Conservation law

A conservation law is a statement used in physics that says that the amount of something does not change in time. That thing could be as simple as mass or charge, or something that has to be calculated, like energy, or angular momentum.

For example, the "law of conservation of mass" is the conservation law that says that the amount of mass is always conserved, even if it is changed into another form. This means that if the mass of the universe could be measured right now, its mass would be known tomorrow because it will not change.