Hardening sprint
An iteration dedicated to stabilisation rather than new features, clearing bugs, finishing integration, and final testing before a release. Frequent hardening sprints are a smell: they indicate quality is being deferred rather than built in continuously, which is why SAFe and most mature teams treat a standing hardening sprint as something to engineer away.
A hardening sprint, a dedicated window to fix bugs, pay down debt, and stabilise before a release, is a useful safety valve and a warning sign at once. Occasionally it's the right call before a high-stakes launch. Routinely needing one means quality is being deferred rather than built in, and the "hardening" is really catch-up for corners cut earlier. Teams that bake testing and review into every sprint need them far less often.