The Law of Conservation of Matter (or Conservation of Mass, the terms are used interchangeably) states that:

In a chemical reaction, the sum of the mass of the reactants equals the sum of the mass of the products.

Another formulation of the Law is expressed as stoichometric balance, that is:

The number of atoms of a particular species in the reactants must equal the number of those atoms in the products.

The Law of Conservation of Matter breaks down for nuclear processes, where the equivalence of matter and energy, and hence conservation of energy, applies.