Canonicity

Canonicity, or canon, was a concept applicable in several different contexts.

Canonicity was firstly of interest to the Time Lords. 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)

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)

According to one account, the concept of canonicity could also be applied to works of fiction; 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)

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. That the Time Lords (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.