But it's not just applicable to lousy software. Even great software is never perfect and making tradeoffs about the design or architecture, or even just the timing of when different components, refactorings, etc. are implemented can be reasonably considered tech debt.

Debt is a liability that you typically need to make payments against; preferably at predictable intervals and of predictable amounts.

Every known (and significant) bug, that isn't fixed, is tech debt. Presumably something has to be done about the effects of the bug and that something has a cost. Fixing the bug then is paying down the associated debt.