Wiki · Reference entry
Regression
Something that used to work quietly stops working.
Familiar from ordinary software, and worse here, because the guarantees were never explicit. A requirement discovered in month two gets satisfied in month two, then broken in month five by a change nobody connected to it, and nothing re-checks it because nothing recorded that it mattered.
Why it matters
The fix is a record of what has to stay true, not more testing at the end.