When a body is moving at a very high velocity, particularly approaching the speed of light, its mass is referred to as relativistic mass. According to Einstein's theory of relativity, as an object's velocity increases, its mass appears to increase as well.
The concept of relativistic mass arises from the equation E = mc², where E represents the total energy of an object, m is its rest mass (mass at rest), and c is the speed of light in a vacuum. As an object accelerates and its velocity approaches the speed of light, its energy (E) increases. Since energy and mass are related, the mass of the object appears to increase as well.
However, it's important to note that the term "relativistic mass" is less commonly used in modern physics. Instead, physicists now typically use the concept of "rest mass" or "invariant mass," which remains constant regardless of an object's velocity. The increase in energy with velocity is attributed to the increase in kinetic energy rather than an actual increase in mass. Thus, the concept of relativistic mass is seen as less fundamental and more of a historical perspective.