noun
rest mass
Physics term for the mass of a particle when it is at rest relative to the observer.
光子の静止質量はゼロである。
The rest mass of a photon is zero.
質量 is general 'mass', while 静止質量 specifically means 'rest mass' in relativity.
Compound of 静止 (rest, stationary) and 質量 (mass), a direct translation of the English physics term 'rest mass'.