Whatever can be pointed at and treated as one. The most general noun there is, which is why it is used when nothing narrower fits.
Old English þing (a meeting, an assembly, and by extension the matter discussed there), from Proto-Germanic *thingam; the most general noun began as a specific gathering before it widened to cover whatever the gathering was about.
The word kept in reserve for whenever a narrower name will not fit. It promises almost nothing about what is named, only that it can be pointed at and counted as one.