WordNet (r) 3.0 (2006):

signal-to-noise ratio
    n 1: the ratio of signal intensity to noise intensity [syn:
         signal-to-noise ratio, signal-to-noise, signal/noise
         ratio, signal/noise, S/N]

The Jargon File (version 4.4.7, 29 Dec 2003):

signal-to-noise ratio
 n.

    [from analog electronics] Used by hackers in a generalization of its
    technical meaning. ?Signal? refers to useful information conveyed by some
    communications medium, and ?noise? to anything else on that medium. Hence a
    low ratio implies that it is not worth paying attention to the medium in
    question. Figures for such metaphorical ratios are never given. The term is
    most often applied to Usenet newsgroups during flame wars. Compare 
    bandwidth. See also coefficient of X, lost in the noise.


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

signal-to-noise ratio
SNR
S/N ratio

   1.  (SNR, "s/n ratio", "s:n ratio") "Signal"
   refers to useful information conveyed by some communications
   medium, and "noise" to anything else on that medium.  The
   ratio of these is usually expressed logarithmically, in
   decibels.

   2.  The term is often applied to Usenet
   newsgroups though figures are never given.  Here it is quite
   common to have more noise (inappropriate postings which
   contribute nothing) than signal (relevant, useful or
   interesting postings).  The signal gets lost in the noise
   when it becomes too much effort to try to find interesting
   articles among all the crud.  Posting "noise" is probably the
   worst breach of netiquette and is a waste of bandwidth.

   [Jargon File]

   (1996-01-29)