The Collaborative International Dictionary of English v.0.48:

Signal to noise ratio \Sig"nal to noise" ra`ti*o\, Signal to
noise \Sig"nal to noise"\, n. (Electronics, Information science,
   Experimental science)
   The ratio of the intensity of a signal[2] to the background
   noise detected by a measuring instrument, especially in a
   communications channel; the higher the ratio, the more
   accurately the information contained can be interpreted. The
   term is applied not only to human communications, but to the
   detection of information in any system that is being studied
   to gain knowledge; as, the signal-to-noise ratio of light
   signals in older optic fibers drops to an unusable level
   after several miles..
   [PJC]