Men of Darkness or Men of Shadow was a term used by the Elves and Dúnedain to denote the men opposed to the Free Peoples of the Westlands.
In the earliest times the term was used in a more neutral sense, to describe the men far removed from the light of Aman and not instructed and teached by the Eldar but in later times the term became more and more a label for the peoples of the East and South who were the descendants of men who had served the Dark Lords in the First and Second Age and who opposed - or were believed to oppose - the Realms in Exile, and their allies.
Most of the Easterlings and Haradrim were considered "Men of Darkness" by the Men of the West.