THD+N is the standard single-number audio quality metric. It sums all harmonic distortion products (2nd, 3rd, 4th... harmonics) plus all noise (hiss, hum, interference) and expresses it as a percentage of the total signal. A THD+N of 0.01% means distortion + noise is 80 dB below the signal.
Limitations: THD+N does not distinguish between pleasant distortion (2nd harmonic, which sounds warm) and unpleasant distortion (7th harmonic, which sounds harsh). Two devices with identical THD+N can sound completely different. SINAD (Signal-to-Noise and Distortion ratio) is the inverse expressed in dB — 80 dB SINAD = 0.01% THD+N.