Technical debt
The accumulated consequences of quick decisions, which make every new change to a system take longer.
Glossary · Maintenance & technical quality
Technical debt arises when the quicker solution is chosen over the tidier one: tests are skipped, code is copied, an upgrade is postponed. It is not always a mistake; sometimes the deadline matters more.
Like financial debt, it charges interest. The more of it there is, the longer every change takes and the more often fixing one place breaks another.
The debt does not have to be repaid all at once. It is usually enough to know where it is and to tidy the areas that change most often. Rewriting everything from scratch is worth it only when tidying would cost more.