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].