Originally Posted By: etaoin
you'll also find a boss on a gong.

Then this definition may a.o. refer to this:

boss:
5. Ornament consisting of a circular rounded protuberance (as on a vault or shield or belt).

I was wondering what the definition meant till I looked at those pictures of the gongs.
Shield and gong have similar shapes more or less.With the gong it may also have a sound function?

And 'vault ' could refer to Zed's definition?