The propulsion matrix was a key component of a starship's propulsion system, and could vary from species to species.
In 2154, when Valdore was attempting to convince Senator Vrax that the Romulan drone ship was untraceable – Vrax noted that the Vulcans would be able to trace it to the Romulan Star Empire the minute they scanned the propulsion matrix. (ENT: "United")
The Delta Flyer was able to knock out a Borg cube's main propulsion system by attacking its propulsion matrix in 2376. (VOY: "Collective")