Mount Doom

Historically, this volcano had been known as Orodruin, the Fire-mountain. After his return to Middle-earth from the Downfall of Númenor, Sauron brought the dormant mountain back to life to use in his preparations for war against the Dúnedain, and understanding the dire implications of its new Fire, the Exiles of Númenor named the mountain Amon Amarth, translated 'Mount Doom'. We don't know the precise date that this occurred, but Sauron launched his war in II 3429, so Orodruin must have become known as Mount Doom in the years (or perhaps decades) before this date.

The timeline for this entry shows the period when the name was in use (the mountain itself had existed for far longer, of course).