Signal-to-noise ratio (SNR) is a measure of a receiver's intended signal as opposed to background noise, expressed as a ratio of the average power of each (signal-to-noise, SN). The power (of each) is directly related to the square of its root-mean-square amplitude i.e., the mean of the amplitude squared. Signal-to-noise ratio is also expressed in decibels, as 10 times the log 10 of the power ratio above. As the log of a ratio, zero means "signal equals noise", and greater than zero means the signal is stronger than the noise.
In astrophysics, the signal-to-noise ratio may be used to characterize the possibility of sensing the output of an astrophysical phenomenon above the background signal and distortion. It is generally applicable but is key in all electronic sensors such as CCDs, and radio telescope antennas and receivers. Such SNR is sometimes expressed as parts per million (PPM) of the "normal" signal, and the variation of interest in the signal is also so-expressed. (I assume such a quoted PPM indicates SNR divided by a million, but I also wonder whether sometimes a quoted PPM is for "signal divided by the sum of signal and noise").