The ratio of the power or volume (amplitude) of a signal to the amount of disturbance (the noise) mixed in with it. Measured in decibels, signal-to-noise ratio (SNR or S/N) measures the clarity of the signal in a circuit or a wired or wireless transmission channel. See interference-to-noise ratio.

The Larger the Number, the Better
The greater the ratio, evidenced by a larger number, the less noise and the more easily it can be filtered out. The lowest number is an SNR of 0, which means that noise and signal levels are the same. Although signals contain non-random intelligence and can be isolated and separated, with a 0 SNR, it would be extremely difficult to isolate the signal in real time. It would be more easily accomplished offline.

As a Metaphor
The terms "signal" and "noise" are also used in chat rooms and Usenet discussions to refer to meaningful discourse (signal) versus worthless blather (noise). It also describes the results obtained from a search engine, where the signal (relevant information) is very small compared to the noise (useless information).