From Merriam-Webster:
1 : extended treatment of or attention to particular items explaining without going into detail giving careful attention to detail. 2 : a part of a whole: such as. a : a small and subordinate part : particular planned the wedding down to the smallest detail also : a reproduction of such a part of a work of art.
Sounds like it means all the parts and aspects of what we can hear, when speaking of detail in regards to audio. Somewhere along the line, some got the notion that it means just the tiny clues like sparkle, decay, ambience and sometimes the emotional intent of the artist (when the stars align).
For me, if any detail is missing, the event is incomplete and lacking. That's not saying it has to be shrill, etched or made to stand out, out of proportion to everything else. It's just info, musical info that's missing, which is just another detail of the event. Not having all the details renders the event incomplete and unconvincing.
A great example of what I'm saying can be seen in the thread of the Pro-Ject CD transport. By all accounts, there's more there, there. All descriptions portray a more convincing musical event. It can be hard to pinpoint but suffice to say, that transport is extracting more info (detail) than what traditionally has been done, making for a more convincing presentation.
All the best,
Nonoise