An Etymological Dictionary of Astronomy and Astrophysics
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The time required for the amplitude of a vibrating system to decrease to 1/e of its initial value.
Of an orbiting object, its lifetime in a non stable orbit.

→ decay; → time.

An upper bound on the time scale on which the magnetic field of a system would decay in the absence of any other agent. It is expressed as: τ_μ = R² / μ, where R is the scale size of the system, η the magnetic diffusivity (η = 1 / μσ, where μ is the magnetic permeability and σ the electrical conductivity). For a star like the Sun, τ_μ  ≅ 10¹⁰ years, so a fossil magnetic field could survive for the star's lifetime on the main sequence. For the Earth, τ_μ  ≅ 10⁴ years, so a → dynamo is required to explain the persistence of the geomagnetic field.

→ Ohmic; → decay; → time.