Before the 20th century, the term
matter included
ordinary matter composed of atoms and excluded other energy phenomena such as
light or
sound. This concept of matter may be generalized from atoms to include any objects having
mass even when at rest, but this is ill-defined because an object's
mass can arise from its (possibly massless) constituents' motion and interaction energies. Thus, matter does not have a universal definition, nor is it a fundamental concept in
physics today. Matter is also used loosely as a general term for the substance that makes up all observable
physical objects.