Möbius strip
From Tardis Wiki, the free Doctor Who reference
A Möbius strip, or Möbius loop, was a mathematical shape that had only one side.
To explain Möbian space to Adric, the Fourth Doctor took a strip of paper, twisted one end, and formed a loop, which created a Möbius strip. (PROSE: Conundrum [+]Loading...["Conundrum (novel)"])
The Twist was a colony ship in the shape of Möbius strip. (COMIC: The Twist [+]Loading...["The Twist (comic story)"])