Canonicity

Canonicity, also called canon, was a concept that determined whether or not a certain source (such as a religious or fictional text) could count as a proper part of a greater whole; this concept was used by humans on Earth, whether they be practitioners of Christianity (AUDIO: ) or fans of the fictional adventures of Iris Wildthyme. (PROSE: )

However, many individuals within the Doctor's universe inexplicably questioned the canonicity of either parts of their own lives or of others, (AUDIO:, PROSE: , WC: , TV: ) with the Time Lords even having a Tower of Canonicity. (PROSE:, , )

Religious canon
In 325, the Council of Nicaea (to which the Fifth Doctor bore witness alongside companions Erimem and Peri) saw a gathering of the greatest scholars and leaders of 4th century Christianity to debate and solidify the reaches of biblical canon, showing the concept was applied by humans to religion. (AUDIO: )

Fictional canon
Panda found out about the spin-offs from the Iris Wildthyme series from "super-fan" who "hardly knew what was canonical". (PROSE: )

By Time Lords and other time travellers
Canonicity was seemingly of interest to the Time Lords, as there existed a Tower of Canonicity in the Capitol, twin to the Tower of Likelihood. (PROSE: )

The Sixth Doctor once expressed doubt on whether something to do with Hecate (about which he didn't elaborate) was "canonical". (AUDIO:, TV: )

The information contained on the TARDIS Wiki influenced Lady Peinforte's search for information on the Doctor, and she disbelieved that the Looms were canonical. (PROSE: )

A group of three Retconning Crocodiles once swore on their canonicity to uphold their side of a bargain with Auteur, in that they would fully resurrect him in exchange for his authorship of a cursed abecedarium; however, while true to their bargain, they left Auteur in the crumbling reality he was in. (PROSE: )

The adventures experienced by River Song where she met her adoptive mother, her "sort-of" sisters, and various incarnations of the Master (including the War Master and Missy) were "probably" canonical. (WC: )

When the Fourteenth Doctor inadvertently travelled to the genesis of the Daleks and accidentally created a bootstrap paradox by giving a Kaled scientist the names "Dalek" and "Extermination", he exclaimed that the "timelines and canon [were] rupturing." (TV: )

Behind the scenes

 * In the real world, canon is (as detailed on this page) a concept originally concerning religious texts, and later more widely applied to works of fiction, beginning with the tradition of Sherlock Holmes.
 * That the Time Lords and others such as the Sixth Doctor and River Song, whom are fictional characters, would thus consider canonicity a genuine property of their (fictional) universe, on par with "likelihood", should therefore be read as a borderline-fourth-wall-breaking, metafictional joke.
 * Whilst the song I Am the Very Model of a Gallifreyan Buccaneer contained numerous references to classic Doctor Who serials, the reference to Hecate was distinct in that it was derived from A Girl's Best Friend, the first and only instalment of the spin-off K9 and Company.

Information from invalid sources
Ken Book interviewed Beep the Meep for Doctor Who Magazine, and after Ken asked if Beep had any plans to appear on television, he responded no, explaining it was a "backwards step" that'd be repeating what had already been done thirty years prior in The Star Beast, and ultimately, he didn't consider Doctor Who on TV to be canonical. He also added he was afraid they'd "do [him] in CGI". (PROSE: )