Time And Relative Dimensions In Space - The TARDIS is a time machine and spacecraft in the science fiction television show Doctor Who and its associated spin-offs, often noted for its interior being substantially larger than its exterior. Always use in conjunction with the [doctor-who] tag.
The TARDIS (/ˈtɑːdɪs/, an abbreviation for "Time and Relative Dimension(s) in Space") is a time machine and spacecraft in the British science fiction television programme doctor-who and its associated spin-offs.
A TARDIS is a product of the advanced technology of the Time Lords, an extraterrestrial civilization 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 any place in the universe. The interior of a TARDIS is much larger than its exterior ("It's bigger on the inside" is a running trope within the show), which can blend in with its surroundings using the ship's "chameleon circuit". The Doctor's TARDIS usually takes the form of a blue "Police Box" since its chameleon circuit has broken.
Source: TARDIS on the English Wikipedia.