Materials can be classified according to their complex-valued permittivity ε, upon comparison of its real ε ′ and imaginary ε ″ components (or, equivalently, conductivity, σ, when accounted for in the latter). A perfect conductor has infinite conductivity, σ = ∞, while a perfect dielectric is a material that has no conductivity at all, σ = 0 ; this latter case, of real-valued permittivity (or complex-valued permittivity with zero imaginary component) is also associated with the name lossless media. Generally, when ⁠ σ / ωε ′ ⁠ ≪ 1 we consider the material to be a low-loss dielectric (although not exactly lossless), whereas ⁠ σ / ωε ′ ⁠ ≫ 1 is associated with a good conductor ; such materials with non-negligible conductivity yield a large amount of loss that inhibit the propagation of electromagnetic waves, thus are also said to be lossy media. Those materials that do not fall under either limit are considered to be general media.