Armageddon was a term used to denote a great disaster, such as the end of the world.
When the Fourth Doctor predicted the deaths of the four billion people of Earth in the space of a year due to the Fendahl, Adam Colby asked if he was "some sort of wandering Armageddon peddler." (TV: Image of the Fendahl)
The Sixth Doctor described the smell of corpses on board Space Station Camera as "the unholy, unburiable smell of Armageddon." (TV: The Two Doctors)
When the Valeyard stated his intent to kill the Sixth Doctor, Sabalom Glitz said that Armageddon was beckoning him. (TV: The Ultimate Foe)
Thoss described the Keth as "the bringers of Armageddon." (PROSE: St Anthony's Fire)
Sally Lucas admitted it was possible that Armageddon (meaning a nuclear war) had happened when the Cadogan Tunnels went into lockdown. (AUDIO: Rat Trap)