This is the usual name given to an imagined planet within the orbit of Mercury, first proposed in 1859 by Urbain De Vernier to explain irregularities in Mercury's orbit (now understood to be an effect of Einstein's Theory of Relativity). Such a planet is observed in Donald W Horner's fanciful By Aeroplane to the Sun: Being the Adventures of a Daring Aviator and His Friends (1910), and it became the setting of several science fiction stories, including R F Starzl's "The Terrors of Aryl" (March 1931 Wonder Stories) (wherein the planet is named Aryl), Leslie F Stone's "The Hell Planet" (June 1932 Wonder Stories), Ross Rocklynne's "At the Center of Gravity" (June 1936 Astounding), and Leigh Brackett's "Child of the Sun" (Spring 1942 Planet Stories). Naturally enough, the planet is typically depicted as extremely hot and filled with perils for unwary travelers; atypically, Edgar Wallace borrowed the name for his Earthlike Counter-Earth in Planetoid 127 (4 September-23 October 1924 The Mechanical Boy; as title story of coll 1929; 1986).
As numerous astronomical observations have failed to confirm the existence of any large bodies within Mercury's orbit, recent authors do not describe such a planet, and after Gene Roddenberry employed its name for the extrasolar planet inhabited by Mr. Spock's people, that has become the planet Vulcan most familiar to sf readers. [GW]