SQM
Also called: sky quality meter, mag/arcsec², sky brightness.
Sky brightness in magnitudes per square arcsecond — higher numbers are darker (22 is pristine, 18 is a city).
A Sky Quality Meter measures how bright the zenith sky is. The number works like a magnitude: a reading of 21.5 means each square arcsecond of sky glows like a star of magnitude 21.5. Pristine sites read about 21.9–22; suburbs 19–20; city centres 18 or below.
In AstroPilot
AstroPilot shows the SQM value behind each Bortle class, estimated from the light-pollution atlas or entered by you.