For an electromagnetic field mode varying sinusoidally with time at a given frequency, the propagation constant is the logarithmic rate of change, with respect to distance in a given direction, of the complex amplitude of any field component.

Note: The propagation constant, λ, is a complex quantity given by λ = α +iβ, where α, the real part, is the attenuation constant and β, the imaginary part, is the phase constant.

Source: from Federal Standard 1037C