In special relativity, **rest mass** is an obsolete term used to describe what is today simply referred to as mass. Relativistic mass *m _{r}* (formerly called "mass", but now called energy) increases with velocity (

*v*), and the rest mass

*m*is the inertial mass at

*v=0*, which corresponds to the classical notion of

*mass*.

When an object is travelling at a velocity *v* relatively to an inertial reference frame, then the relativistic mass increases according to:

*special relativity*.

The terms *rest mass* and *relativistic mass* can be still be found in elementary textbooks and popularisations of relativity, but their use is depreciated in graduate-level physics.