Relativistic momentum and relativistic mass are concepts in Einstein's theory of special relativity that describe how momentum and mass change at velocities close to the speed of light. Relativistic momentum is defined as an object's mass multiplied by its velocity, where mass increases as an object's velocity approaches the speed of light. Relativistic mass represents the inertia of an object, or how difficult it is to accelerate, which also increases at relativistic speeds due to the object's increased energy.