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Rest mass (invariant mass) or rest energy

body's intrinsic mass (or equivalent intrinsic energy), motion-independent (Lorentz invariant); equals total mass (or energy) when at rest

Pronunciation
/rɛst mas ( ɪnˈvɛːrɪənt mas ) ɔː rɛst ˈɛnəʤi/
/rɛst mæs ( ˌɪnˈvɛriənt mæs ) ɔr rɛst ˈɛnərʤi/