A jacket that is naturally designed with a low and wide-cut neckline, not covering the shoulders or upper chest area, leaving them bare.
In regards to the tag usage on Danbooru, compared to other specific "off-shoulder" tags, this tag is very infamous for being frequently misused among the userbase for open or partially-open jackets that are worn hanging off the upper arms, regardless of how they were designed to be worn.
Jackets which qualify for this tag include post #4840348, post #4562464, post #4068521, post #4032309, post #3912213, post #3645458, post #4234872, and post #4257820, since close inspection shows that these jackets are naturally designed with a low neckline by default, meaning they will leave the shoulders bare when worn correctly.
Jackets that do not qualify for this tag include post #3694235, post #3629456, post #3643155, post #4226347, and post #4072640, since these are merely regular jackets that are worn hanging off the upper arms instead of the original way these jackets are meant to be worn (on the shoulder), despite the jacket not being designed with a low neckline in mind. The jackets worn by Taihou (in her Enraptured Companion outfit), Botan, and Towa are some examples of jackets that do not qualify for this tag.
If unsure whether or not the clothing in question qualifies as an off-shoulder jacket, use the off shoulder tag instead.
This tag implicates off_shoulder (learn more).