A Jefferies tube
is a service corridor found in Federation starships
, with equivalents existing in many other Star Trek
ships. They vary in size from standing height to crawlspaces, traversing the ship both horizontally and vertically. Jefferies tubes are mainly used to repair and maintain systems that can't be reached from the normal corridors, but they can also be used to move through the ship in emergencies.
In TOS, there were a few repairs that could only be made by crawling into a service tube in the engineering section. In TNG and beyond, the use of Jeffries tubes became a staple for the writers, to the point that some of these service "crawlways" were as wide as the regular corridors, and their total volume surely rivaled that of the main halls and compartments.