From TARDIS Index File, the free Doctor Who reference database.
The TARDIS Index File was a searchable database discovered by the Doctor's companions Nyssa and Tegan in Episode 1 of the 1982 serial, "Castrovalva."
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 and destroy the Doctor. The TARDIS Index File was part of the Master's modifications, and not an actual part of the TARDIS' standard equipment.
The TARDIS Index File website, a collaborative project to create the most definitive, accurate, and accessible encyclopedia and reference database for everything related to Doctor Who, using a database formatted in a WikiWiki structure which visitors can edit, takes its name from this temporary feature of the Doctor's TARDIS.