The TARDIS (Time and Relative Dimension(s) in Space) is a time machine and spacecraft in the British science fiction television programme Doctor Who.
A product of Time Lord technology, 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, which can blend in with its surroundings through the ship's chameleon circuit. In the series, the Doctor pilots an unreliable, stolen, obsolete Type 40 TARDIS, once referred to as a TT Capsule, whose chameleon circuit is faulty, leaving it locked in the shape of a 1950s-style London police box. It was stolen from Gallifrey where it was undergoing maintenance, and the unpredictability of the TARDIS's short range guidance -- that is, relative to the size of the entire Universe -- has often been a plot point in the Doctor's travels.
Although "TARDIS" is a type of craft, rather than a specific one, the Doctor's TARDIS is usually referred to as "the" TARDIS or, in some of the earlier serials, just as "the ship". (In the two 1960s Dalek films, the craft was referred to as Tardis, without the definite article.)
Doctor Who has become so much a part of British popular culture that not only has the shape of the police box become more immediately associated with the TARDIS than with its real-world inspiration, the word "TARDIS" has been used to describe anything that seems bigger on the inside than on the outside. The name TARDIS is a registered trademark of the British Broadcasting Corporation.