RMS level
The root mean square of a signal over a time window – a measure of how much energy the sound carries, as opposed to how high its peaks reach.
Square the samples, average them, take the square root. For a sine wave the result is 0.707 of the peak, or 3 dB lower, so AES17 adds 3 dB to make a full-scale sine read 0 dBFS. When two meters disagree by exactly 3 dB, they usually disagree about that convention, not about the audio.
RMS tracks perceived loudness far better than peak level, and the gap between the two is the crest factor. Compressors have long offered a choice of reacting to peak or RMS. As a measure of how loud something actually sounds, though, RMS has been overtaken by LUFS, which also accounts for the ear's uneven sensitivity across the frequency range.