SARDIT was a term used by the Seventh Doctor in reference to the opposite of a TARDIS, that instead of "Time And Relative Dimension In Space" it was "Space And Relative Dimensions In Time."
The Doctor's TARDIS became a SARDIT after colliding with the Time Scaphe immediately following an invasion by the creature later called the Process. As a result, the TARDIS was transformed into a multi-dimensional city that curved into itself, reminiscent of a Dyson sphere. Pieces of the TARDIS were either scattered across the city or were transformed into it.
Due to the reversed nature of the dimensions, three time zones, Past, Present, and Future, occupied the city, separated by mercury streams. It was possible, though, to cross the streams and enter into another time to see what would happen. Due to the fluidity of time within the city, the Future was capable of being changed upon interactions between the Present and Past as the Blinovitch Limitation Effect was non-existent.
The Doctor later returned his SARDIT/TARDIS to normality with the Banshee Circuits and Architectural Configuration. (PROSE: Cat's Cradle: Time's Crucible)