An Etymological Dictionary of Astronomy and Astrophysics

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.

See also: → longitudinal; → mass.