The mass of something which is not moving.

Due to relativistic effects, the mass of an object increases with increased speed, as mass is directly related to energy, connected by the equation E=mc2 (Einstien's equation connecting mass and energy: since the speed of light is constant, E and m are directly proportional). The energy of the object is due to kinetic energy.

The Rest Mass of an object (usually denoted by M0) is equivalent to the energy contained in the object due to chemical bonding, the attraction of electrons to nuclei, and the Rest Masses of the individual sub-atomic particles.

Any object that can exist below the speed of light has a finite rest mass, and an infinite mass at the speed of light, and any object that can exist at the speed of light has zero rest mass and a finite mass at the speed of light.