The acronym TARDIS, coined by Susan Foreman, stands for Time and Relative Dimension in Space. A complex piece of Gallifreyan technology, a TARDIS is a space-time vessel used by the Time Lords. The machines are capable of travelling through time and space by disappearing and materialising somewhere else almost instantly. Key features of a TARDIS include its inside being bigger than its outside and the chameleon circuit, which allows the ship to blend in with the landscape of wherever it landed, so as not to attract unwanted attention. TARDISes also have a degree of sentience, being able to take independent action and developing a profound bound with its user.