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.

The Doctor told Clara Oswald that he had encountered truth fields before, calling it a "quaint" technology.