  The axiomatic definition of mass in classical mechanics, outlined by Mach in
the second half of 19th century and improved by several authors, is simplified
and extended to the theory of special relativity. According to the extended
definition presented here, the mass of a relativistic particle is independent
of its velocity and coincides with the rest mass, i.e., with the mass defined
in classical mechanics. Then, force is defined as the product of mass and
acceleration, both in the classical and in the relativistic framework.
