Retimer
Beginner
A small chip partway along a long connection. It cleans up the signal and sends it on, so it can travel farther.
Novice
A chip that receives a fast PCIe signal, recovers the data, and transmits a fresh copy. It is used when the wire is too long or lossy for the signal to arrive cleanly.
Expert
A protocol-aware, software-transparent physical-layer device that splits a channel into two segments, each with its own loss budget. Unlike a redriver (an analog amplifier), it resets jitter and noise, at the cost of a few nanoseconds of latency, power and board area.