In J. R. R. Tolkien's legendarium, the Lonely Mountain is a mountain northeast of Mirkwood. It is the location of the Dwarves' Kingdom under the Mountain and the town of Dale lies in a vale on its southern slopes.
In The Lord of the Rings, the mountain is called by the Sindarin name Erebor. The Lonely Mountain is the destination of the protagonists, including the titular Hobbit Bilbo Baggins in The Hobbit, and is the scene of the novel's climax.
The mountain is the goal of Bilbo's psychological quest in The Hobbit; scholars have noted that it and The Lord of the Rings are both structured as quests to a distant mountain, but that the quests have very different motivations. Further, the mountain is a symbol of adventure in The Hobbit, and of Bilbo's maturation as an individual, while to the Dwarves, it stands for the gain of beauty in return for loss of life.
Erebor stands hundreds of miles from the nearest mountain range. Tolkien's rendering of Thrór's map in The Hobbit shows it with six ridges stretching out from a central peak that was snowcapped well into spring. The whole mountain is some ten miles in diameter; it contains an immense wealth of gold and jewels.
Erebor becomes the home of the Folk of Durin, a clan of Dwarves known as the Longbeards, after they are driven from their ancestral home of Khazad-dûm. In the latter days of the Third Age, this Kingdom under the Mountain holds one of the largest dwarvish treasure hoards in Middle-earth. Dale, a town of Men built between the two southern spurs of Erebor, grow in harmony with the dwarves. The Kingdom under the Mountain is founded by Thráin I the Old, who had discovered the Arkenstone there. His son, Thorin I, leaves the mountain with much of the Folk of Durin to live in the Ered Mithrin (Grey Mountains) on account of the great riches to be found in that range. After dragons plunder their hoards, the Longbeards, led now by Thrór, a descendant of Thorin, return to Erebor to take up the title King under the Mountain.