Truth Field

A Truth Field was an energy field within which it was impossible to tell a lie. The Doctor encountered one near the bell tower in the town of Christmas on Trenzalore. The field emanated from a residual crack in space-time along with the Question, both transmitted by the Time Lords from outside the universe with the purpose of verifying the Doctor's identity. Being clever, the Doctor could circumvent this; he lied by telling the truth to kill a Wooden Cyberman.

The Eleventh Doctor told Clara Oswald that he had encountered Truth Fields before, calling it a "quaint" technology. (TV: The Time of the Doctor)