Recursion

Recursion is a concept of ideas and concepts repeating within themselves, this concept, specifically "If we had an index file we could look it up in the index file under 'Index file'", which gave Tegan Jovanka the idea to search the TARDIS index file using the letters IF. (DW: Castrovalva) The Master tricked the Doctor into materialising his TARDIS around the Master's, creating a dimensionally recursive loop. (DW: Logopolis) The TARDIS once materialised inside itself. (DW: Space, Time)
 * See also Space loop