An artron inhibitor was a device that inhibited the block-transfer equations that made reconstitution and regeneration of a TARDIS's outer plasmic shell possible, limiting their invulnerability while also draining the artron energy they needed as a power source. As an apparent side-effect, it also disabled the Time Lords' own ability to regenerate.
When the Eighth Doctor landed in 1898, Cuthbert Simpson was using an inhibitor. It locked onto Compassion, the Doctor's current TARDIS, when she materialised in the area, draining her of her artron energy. Until the inhibitor was deactivated, Compassion was forced to take Susan Seymour as a host to maintain her own stability.
After Simpson was injured falling from a great height, he was unable to regenerate due to the inhibitor's effect. He managed to survive but it took him fifty years to recover.
The Doctor eventually found the inhibitor hidden among Richard Harries' equipment and destroyed it. (PROSE: The Banquo Legacy)