In the StarTrek universe, a ''Jeffries tube'' is a narrow conduit, just large enough for a human-sized being to crawl through, that provides access to the internal systems of a starship or other facility.  When crew members need to fix things, they crawl into the tubes with some futuristic tools and go to work.

When characters are lucky, they have to crawl into the tubes and work in close quarters with attractive members of whatever gender they like to rub up against. If they are not lucky, the alien or particle of the week traps them in the tube and eats them.

The name comes from the name of the TV series production designer who created the set. 
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See also StarTrekSystems