time and relative dimensions in space

T.A.R.D.I.S. It refers to the Gallifreyan time and space travel capsule which is dimensionally transcendental (bigger on the inside). The most common example is the one used by the Gallifreyan Time Lord known as "The Doctor", that is permanently disguised as a 1960s police telephone box.

Susan: I made up the name TARDIS from the initials - Time And Relative Dimensions In Space.