TARDIS is short for Time and Relative Dimensions in Space, it is a mode of transport used by the Time Lords of Gallifray and capable of transport anywhere and anywhen in the universe (or even outside it). These devices are larger on the inside then the outside (thanks to dimensional engineering) and capable of changing their outer appearance to blend in with the surroundings of their destination.

The Doctor (see Doctor Who) stole a Type 40 TARDIS from his people and took off round the universe, soon after leaving the chameleon circuit got stuck and the TARDIS was no longer able to change its appearance from that of a 1970s type British police telephone box.

In a BBC book, the Doctor's original TARDIS was destroyed and one of his companions mutated in to a new one. Being the most advanced TARDIS in the universe she and the Doctor have to flee from the Time Lords who want to capture her and mate her with their bull TARDIS's to form a new race of time machines to help them in their war with The People.

Without wishing to be pedantic... oh, all right, maybe I do, but anyway, I think it's worth mentioning that TARDIS actually stands for Time And relative Dimension In Space (singular dimension).

In the original episode, 'An Unearthly Child', The Doctor's granddaughter Sarah tells someone "it stands for Time And Relative Dimension In Space". Okay, so ever since then it's been more than one Dimensions, but that first instance is canonical. Sorry, guys.

The TARDIS was notable for being bigger on the inside than on the outside.

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wertperch points out that http://www.bbc.co.uk/cult/doctorwho/tardiscam/intro.shtml states: "TARDIS, of course, stands for Time And Relative Dimensions In Space. Or Time and Relative Dimension in Space, if you're a purist." Vindicated at last.

Tardis is a limited-access UNIX service hosted by the University of Edinburgh. The Tardis project began in August 1988, and ran a public service until August 1991 based on old GEC hardware discarded by the university. Since late 1993, they have had a new service running on an assortment of new bits of discarded kit.

Tardis is a resource available to those affiliated with Edinburgh University. The Tardis Charter sets out the basis on which they operate this service. University undergraduate students (2nd year and above), postgraduates, members of staff, and in exceptional circumstances members of the public may apply for an accountt. Tardis is jointly administered by a distributed group known as the timelords who consist of students and ex-students of the University of Edinburgh.

The control room of the Tardis also had one very neat property - energy weapons did not work in its confines.

This provided a nail-biting end to Earthshock, where the Doctor goes hand-to-hand with the leader of the Cybermen and destroys it, via his only remaining momento of Adric - Adric's gold badge.

The statement by TardisX that weapons don't work inside the TARDIS control room is completely false. The Doctor surprised the cybermen leader from behind with the gold star (gold is lethal to them) and rubbed it into his suit, causing him to stumble around the control room firing his weapon repeatedly into the TARDIS console. This damaged the console preventing the Doctor from saving Adric.

Furthermore, the answer previous question "Can you fit a TARDIS inside another TARDIS?" is YES. This occured in the episode Logopolis, in which the Master's TARDIS materialized in the same location as the Doctor's TARDIS, and ended up inside it. This made for some very strange scenes where the Doctor and his companions end up going into a TARDIS inside of a TARDIS. There was another strange issue with the TARDIS in this episode, being that it was in fact so large inside that there were areas that the doctor hadn't been to in a very long time, and were literally crumbling from age.