Visual stabiliser circuit
From Tardis Wiki, the free Doctor Who reference
The visual stabiliser circuit controlled the TARDIS' outward appearance. Its removal rendered the ship invisible. (TV: The Invasion)
Its relation to the Chameleon circuit, whether they were one and the same, is unknown. Furthermore, this circuit's relation to the TARDIS's ability to become invisible is unknown. (TV: The Impossible Astronaut, Day of the Moon)
