matter

In physical science, mass refers to the degree of acceleration a body acquires when subject to a force: bodies with greater mass are accelerated less by the same force. One says the body of greater mass has greater inertia. The mass of an amount of matter in a chemical substance is determined in part by the number and type of atoms or molecules it contains, and in part by the energy involved in binding it together (which contributes a negative "missing mass," or mass deficit). According to special relativity, energy also has mass according to the principle of mass–energy equivalence as exemplified in the process of nuclear fusion and the bending of light.[1]
Inasmuch as energy is conserved in closed systems in relativity, the relativistic definition(s) of mass are quantities which are conserved also; they do not change over time, even as some types of particles are converted to others.
In everyday usage, mass is commonly confused with weight. But, in physics and engineering, weight means the strength of the gravitational pull on the object; that is, how heavy it is, measured in units of newtons. In everyday situations, the weight of an object is proportional to its mass, which usually makes it unproblematic to use the same word for both concepts. However, the distinction between mass and weight becomes important for measurements with a precision better than a few percent (due to slight differences in the strength of the Earth's gravitational field at different places), and for places far from the way surface of the Earth, such as in space or on other planets.