Concept Drift
A change in the relationship between inputs and the correct answer, so the same input now means something different.
Written changing: what used to be a spam signal is now how everyone writes, because spammers adapted. Reweighting can't help, because the thing you learned is no longer true — this is the case that needs a new model rather than an adjustment.
It's also the expensive one, for two reasons. Input monitoring cannot see it: the inputs can be perfectly stable while the answer changes underneath them, so every drift dashboard stays green.
And detecting it needs labels, which arrive late. A churn label takes 90 days and a default takes years, so by the time you can measure the thing that matters, a quarter has gone. The workable substitutes are proxies — the prediction distribution, the confidence distribution, the unseen-category rate — plus a small, slow, sampled labelled feedback path, which is the only instrument that actually sees it.