Note 1 to entry: For an isotropic medium, the relative permittivity is a scalar quantity; for an anisotropic medium, it is a tensor quantity.

Note 2 to entry: In the case of constant fields and alternating fields of sufficiently low frequency, the relative permittivity of an isotropic or quasi-isotropic dielectric is equal to the ratio of the capacitance of a capacitor, in which the space between and around the electrodes is entirely and exclusively filled with the dielectric medium, to the capacitance of the same configuration of electrodes in a vacuum. However, this use is not recommended.

Note 3 to entry: In practical engineering, it is usual to employ the term "permittivity" when referring to relative permittivity, but this use is deprecated since "permittivity" is a synonym for "absolute permittivity".