Demon!Dean is a nickname used in fandom to refer to a specific portrayal of Dean in fanfiction when he is either possessed by a demon or, after the revelation in 3.09 Malleus Maleficarum that humans become demons in Hell, he himself is transformed into a demon. The latter was first popularized by post-season 3 AU fic in which Dean is turned into a demon before he escapes Hell, and later became standard to any fic dealing with the canonical fallout of season 9.
Demon!Dean eventually became a canon character. After Dean accepts the Mark of Cain from Cain himself in Season 9, it begins changing him, granting him supernatural powers, ramping up his bloodlust, and making him addicted to using the First Blade. The Mark then resurrects him as a demon when he is killed by Metatron in 9.23 Do You Believe in Miracles?. Previously, Demon!Dean had appeared as Dean's worst nightmare in 3.10 Dream a Little Dream of Me.