TARDIS information system

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

Articles in the database could be accessed by typing in a two-letter initialism 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 turned out had modified the Doctor's TARDIS using 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 otherwise have existed. (TV: 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. (TV: Frontios) This event would actually be witnessed by the Doctor four regenerations later, with Rose Tyler. (TV: The End of the World)