In Star Trek technobabble, Jefferies Tubes were these crawlspaces between decks and all around any place where theoretically somebody sometime would have to do something more than stare at a screen and punch a bunch of buttons. Supposedly the average starship in the 25th century would be mostly automated and self-maintained. However eventually either something would get really broke, or the something that was supposed to fix the something would get really broke, so a normal average human (or alien) would have to climb up in there and actually get his hands dirty. Scotty knew about his Jefferies Tubes intimately, as did Geordi LaForge and Chief Miles O'Brien. And I hear B'Elanna Torres got to know Tom Paris intimately in the Jefferies Tube.

Jefferies Tubes were originally concieved by Star Trek: The Original Series art director Matt Jefferies, who logically figured that no engineer worth his weight in scotch would venture into space on any ship that didn't have some equivalent of a car hood. These tubes also come in handy for Star Trek writers, because whenever aliens try to take over the ship, the characters have some place to run.

Update: April 6th, 2005
mauler says Re: Matt Jefferies, Jefferies Tube - These were not named *by* Jefferies - they were named by other people *in honor* of him, cause he designed the original USS Enterprise model. Please correct when you have time.