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: The Council of Nicaea) or fans of the fictional adventures of Iris Wildthyme. (PROSE: From Wildthyme with Love)

However, many individuals within the Doctor's universe actually questioned the canonicity of either their own adventures or those experienced by others, (AUDIO: Doctor Who and the Pirates, PROSE: Lady Peinforte, WC: The Story of the Diary of River Song) with the Time Lords even having a Tower of Canonicity. (PROSE: The Blue Angel)

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: The Council of Nicaea)

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

The information contained on the TARDIS Wiki influenced Lady Peinforte's search for information on the Doctor and the canonicity of information she could find. (PROSE: Lady Peinforte)

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 Blue Angel)

The Sixth Doctor once expressed doubt on whether something to do with Hecate (about which he didn't elaborate) was "canonical". (AUDIO: Doctor Who and the Pirates, TV: A Girl's Best Friend)

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: The Story of the Diary of River Song)

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.