Loudness is the perceived intensity of sound — how loud it actually seems to a listener — which is a different thing from peak level. Two clips can hit the same peak yet feel very different in loudness because perception depends on sustained energy and on frequency: the ear is more sensitive in the midrange. Modern measurement captures this with K-weighting and gating (ITU-R BS.1770), reporting the result in LUFS. Getting loudness right is what platform normalization is about: deliver near a target and your content sits at a consistent level beside everything else, instead of being turned up or down or jumping in volume between programs and ads.
