Trace buffer
Beginner
A small memory inside the chip that records recent signals, like a flight recorder, so engineers can see what happened before a failure.
Novice
A small memory inside the chip that keeps recording a chosen set of internal signals and freezes when a trigger condition occurs, so engineers can read out what happened just before a failure through the debug port.
Expert
Its size sets how many signals and how many clock cycles can be seen, so teams choose signals carefully, compress the data and share buffers between blocks. It works best for logic bugs that can be made to happen again.
Explained in After tapeout (Extras).
See also: Post-silicon validation, IJTAG (IEEE 1687).