Loudness is the subjective way we perceive Sound Pressure.
Perceived loudness is loudness. As above, loudness is in itself a perception. Perceived loudness is kind of a redundant term.
LUFS stands for Loudness Units Relative to Full Scale. Full Scale being the scale on which we use to measure the volume of a given signal. Where 0 dBFS (decibel full scale) represents the highest sample value on the scale and anything above that is considered clipping, which can result in distortion.
When you hear about LUFS values, they are simply approximations of the loudness of a signal relative to digital full scale. So if one signal has a higher LUFS than another, it’s approximated that it’s a louder signal.
Just because Spotify normalises to -14 LUFS, doesn’t mean you have to!
LKFS (aka LUFS) Wikipedia Article
