Epsilon (uppercase Ε, lowercase ε or lunate ϵ; έψιλον) is the fifth letter of the Greek alphabet, corresponding phonetically to a mid<!-- not close-mid, see Arvanti (1999) - Illustrations of the IPA: Modern Greek. --> front unrounded vowel.
In mathematics, the epsilon numbers are a collection of transfinite numbers whose defining property is that they are fixed points of an exponential map.
The relative permittivity of a material is its (absolute) permittivity expressed as a ratio relative to the permittivity of vacuum.
The physical constant (pronounced as "epsilon nought"), commonly called the vacuum permittivity, permittivity of free space or electric constant, is an ideal, (baseline) physical constant, which is the value of the absolute dielectric permittivity of classical vacuum.