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  1. A measure of the rate of attenuation per unit length; the rate of flux-density (or power) reduction as energy (visual, electromagnetic, acoustic) propagates from its source. Also called attenuation factor, decay constant. Compare attenuation coefficient. For free-space transmission of radar frequency energy, the attenuation constant is usually expressed in decibels per mile or kilometer (db/mi or db/km).
  2. Specifically, of a traveling plane wave at a given frequency, the relative rate of decrease of amplitude of a field component (or of voltage or current) in direction of propagation in nepers per unit length.