relativistic limit

In general, the energy of an individual particle is related to the sum of its mass energy and its kinetic energy by Einstein's equation E2 = p2c2 + m2c4, where p is the particle's momentum, m is its mass, and c is the speed of light. When a particle is moving very close to the speed of light, the first term (p2c2) is much larger than the second (m2c4), and for all practical purposes the second term can be ignored. This approximation—ignoring the mass contribution to the energy of a particle—is called the "relativistic limit."