TARDIS

TARDIS is short for Time And Relative Dimension In Space.

It a an highly advanced time machine with a sentient mind, a product of the incredible technology of the Time Lords. the most famous thing about the TARDIS is that is a bigger on the inside which is a part of the transdimensional engineering that Time Lords have developed and perfected and it is known to only them. It can retain any form through its chameleon circuit and use it to disguse itself so it can better blend in with its surroundings.

'TARDISes are grown, not made.' The Doctor explains how the only place the TARDIS'es grow is Gallifrey itself.