For a given frequency of radiant energy, a complex quantity describing the medium through which the radiation is propagating. The real part is the specific attenuation, usually measured in decibels per unit path length, and the imaginary part is the phase constant or change in phase, in radians per unit path length.
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.