Definition
Doing more work in the same time, or the same work in less time, by shortening rests or adding sets inside a fixed window.
Example: 15 minutes, as many rounds of 8 push-ups and 8 rows as possible with good form. Next week the target is one more round. Progress is obvious and the session fits a lunch break.
It suits general fitness and fat-loss clients and builds work capacity. It does not build maximal strength, and form drops as the clock runs, so keep the movements simple and the loads moderate.
Density can also be progressed the other way: same rounds, less time.