Defect density
The number of confirmed defects per unit of size, per thousand lines of code, per story point, or per module, used to compare quality across components and track it over time. Defect density is a relative signal, not an absolute target: a rising density in one area flags where to invest in tests or refactoring, not a number to game down.
Defect density, defects per unit of size, whether KLOC, story points, or modules, is most useful as a comparison, not an absolute: it points to where bugs cluster so attention and rework go to the riskiest code. The caveat is that it's only as honest as its inputs, inconsistent size measures or under-reported defects make it meaningless. Used carefully, it feeds risk-based testing; used naively, it becomes a number teams learn to game.