The Collaborative International Dictionary of English v.0.48:

Attenuation \At*ten`u*a"tion\, n. [L. attenuatio: cf. F.
   att['e]nuation.]
   1. The act or process of making slender, or the state of
      being slender; emaciation.
      [1913 Webster]

   2. The act of attenuating; the act of making thin or less
      dense, or of rarefying, as fluids or gases.
      [1913 Webster]

   3. The process of weakening in intensity; diminution of
      virulence; as, the attenuation of virus.
      [1913 Webster]

WordNet (r) 3.0 (2006):

attenuation
    n 1: weakening in force or intensity; "attenuation in the volume
         of the sound" [syn: attenuation, fading]
    2: the property of something that has been weakened or reduced
       in thickness or density

The Free On-line Dictionary of Computing (30 December 2018):

attenuation

    The progressive reduction in amplitude of a
   signal as it travels farther from the point of origin.

   For example, an electric signal's amplitude reduces with
   distance due to electrical impedance.  Attenuation is
   usually measured in decibels [per metre?].

   Attenuation does not imply appreciable modification of the
   shape of the waveform (distortion), though as the signal
   amplitude falls the signal-to-noise ratio will also fall
   unless the channel itself is noise free or the signal is
   amplified at some intermediate point(s) along the channel.

   ["Networking Essentials, second edition",
   Microsoft Corporation, pub. Microsoft Press 1997].

   (2003-07-29)