relativistic mass

One prediction of special relativity is that, the faster an object already is, the more difficult it is to accelerate it even further. One consequence of this is that it is impossible to accelerate a material object to the speed of light: The faster the object already is, the more force has to be used to increase its speed, and close to the speed of light, this effect becomes so strong that, finally, one would have to use infinite force to effect the final, decisive acceleration.

Traditionally, in classical physics, the resistance of an object to changes of its state of motion is its (inertial) mass. The relativistic mass of an object is defined in the same way, and the value an observer measures for this relativistic mass increases as an object moves faster and faster relative to that observer.