Signal-to-noise ratio (SNR)
How much stronger the wanted signal is than the noise beneath it, expressed in decibels.
Every chain has a noise floor: thermal noise in resistors, tape hiss, quantisation noise. SNR measures the distance from the reference level down to it. A compact cassette managed roughly 50–60 dB, which explains why Dolby B, introduced in 1968, sold so well. In digital audio each bit buys about 6 dB, so 16 bits give around 96 dB.
Spec-sheet figures are often A-weighted, which discounts noise in the ranges the ear is least sensitive to and flatters the number by a few dB. Comparing a weighted figure with an unweighted one is a time-honoured way to win a forum argument. Related but not the same: dynamic range and THD.