LUFS
Loudness Units relative to Full Scale — a measure of how loud something is perceived to be, not just how high its peaks reach.
Peak levels say little about perceived loudness: a heavily compressed track and a dynamic one can share the same peak and sound wildly different. ITU-R BS.1770 (2006) defined a measurement that weights frequencies roughly like the ear and averages over time, and EBU R 128 (2010) made −23 LUFS the target for European broadcast. In the US the same unit is called LKFS.
The biggest effect was on music. Spotify and YouTube normalise playback to around −14 LUFS, Apple Music to −16, so a master squashed to the limit simply gets turned down — and then sounds flat next to one that kept its dynamics. The loudness war was lost to an algorithm.