Propagation constant
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