The concept of mass is usually introduced through Newton’s Laws. Associated to the concepts of force and acceleration in the second law of Newton f = m a , valid, it is affirmed, in inertial frames. Since Newton's laws does no define force and inertial frame in a logical unobjectionable manner (Eisenbud, 1957) the concept of mass must be introduced in a way consistent with previous established concepts that we assume a priori. The concepts of space and time permits the introduction of two concepts that define motion – velocity and acceleration. If we admit symmetry properties to space the mass concept emerge as a relation of distances of the centres of two spheres with mass m and mp (standard mass) and another point designated mass centre, at rest, the spheres describe a central motion. Mass is defined by motion. The spheres gravitate around the mass centre with accelerations in the same relationship of the masses, the relationship of the distances to the mass centre. Initially we admit a classical approximation since time is the same everywhere independently of the frame of reference.