TARDIS information system

The TARDIS Information System (or TARDIS Index File) was a searchable database discovered by the Doctor's companions Nyssa and Tegan from a readout in the Control room.

Articles within the database could be accessed by typing in a two letter acronym for the subject. Entering IF, for example, would bring up the database's Index File. Nyssa noted that using the Index File to search for the Index File was an example of recursion.

It was later revealed that the Master had modified the Doctor's TARDIS via Block Transfer Computation as part of his plan to ensnare the Doctor in Castrovalva. At least some of the TARDIS Index File's contents, such as the entry on Castrovalva, would not normally have existed there. (DW: Castrovalva)

Turlough read aloud a historical account from the index file, telling of Earth's collision with its sun to Tegan in order to purposefully upset her. (DW: Frontios)