A tetradecagon was a fourteen-sided figure.
When trying to understand why the Globe Theatre had a tetradecagonal design, the Tenth Doctor enlisted the help of Martha Jones and William Shakespeare to find other objects and concepts that were arranged around the number fourteen — like sonnets. As it turned out, the tetradcagal shape of the Globe — in fact designed by the Carrionites — was related to the fourteen stars of the Rexel planetary configuration. The odd design allowed the Carrionites to turn the theatre into an energy converter which they hoped to use to destroy human life on Earth. (TV: The Shakespeare Code)
Behind the scenes Edit
The Doctor only actually used the adjectival form of this word in the episode, while Lilith used the noun.