The TARDIS (UK /ˈtɑːdɪs/; US /ˈtɑːrdɪs/; Time And Relative Dimension In Space) is a fictional time machine and spacecraft that appears in the British science fiction television programme Doctor Who and its various spin-offs.
A TARDIS is a product of the advanced technology of the Time Lords, an extraterrestrial civilisation to which the programme's central character, the Doctor, belongs. A properly maintained and piloted TARDIS can transport its occupants to any point in time and space. The interior of a TARDIS is much larger than its exterior. It can blend in with its surroundings using the ship's "chameleon circuit". TARDISes also possess a degree of sapience and provide their users with additional tools and abilities including a universal translation system based on telepathy.
In the series, the Doctor pilots an apparently unreliable, obsolete TARDIS; in the episode "Let's Kill Hitler" (2011), it is designated "TT Type 40, Mark 3. Its chameleon circuit is broken, leaving it stuck in the shape of a 1960s-style London police box after a visit to London in 1963. The Doctor's TARDIS was for most of the series' history said to have been stolen from the Time Lords' home planet, Gallifrey, where it was old, decommissioned and derelict.
The unpredictability of the TARDIS' short-range guidance (relative to the size of the Universe) has often been a plot point in the Doctor's travels. In "The Doctor's Wife", the TARDIS reveals that much of this "unpredictability" was actually intentional on its part in order to get the Doctor where he needed to go as opposed to where he wanted to go.
Doctor Who has become so much a part of British popular culture that the shape of the police box become more immediately associated with the TARDIS than with its real-world inspiration. The name TARDIS is a registered trademark of the British Broadcasting Corporation (BBC)