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 E^{2} = p^{2}c^{2} + m^{2}c^{4}, 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 (p^{2}c^{2}) is much larger than the second (m^{2}c^{4}), 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."