A retimer is a chip used inside some cables and devices to regenerate a high speed digital signal. As data rates climb into the tens of gigabits per second used by USB4 and Thunderbolt, a signal naturally degrades over distance, and a retimer effectively rebuilds a clean version of it partway along the connection.
This is different from a simpler redriver chip, which only amplifies a signal without fully regenerating its timing. Retimers are more capable, and more expensive, which is one reason why longer, high speed active cables tend to cost more than short, basic passive ones.
For everyday users, the presence of a retimer inside a cable is invisible, but its effect shows up as the ability of a longer cable to still reach full USB4 or Thunderbolt speed where a passive cable of the same length would fail. See our guide to USB cables for buying advice.
