An Etymological Dictionary of Astronomy and Astrophysics
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In special relativity theory, the mass of a body when the acceleration is parallel or anti-parallel to velocity: m_l = m₀ / [1 - (v/c)²]^3/2, where m₀ is the → rest mass, v is the velocity, and c the → velocity of light. → transverse mass.

→ longitudinal; → mass.