Nameless One

The people of Gondor, who lived on the borders of Mordor and guarded against Sauron's forces, would by tradition avoid using Sauron's name directly. Rather, he was commonly called simply the Nameless One (or the Nameless Enemy, the Enemy, or the Unnamed) and by extension his Black Land of Mordor was the Nameless Land. (The word 'nameless' here does not imply, of course, that Sauron had no name, but only that his name was not used.)

It is not known whether the Gondorians actually thought that using Sauron's name would bring ill fortune, or avoided it as a simple sign of contempt, but the tradition was clearly well established. Boromir, indeed, at one point refers to Sauron as 'him that we do not name' (The Fellowship of the Ring II 2, The Council of Elrond). The tradition of avoiding Sauron's name appears to have been limited to Gondor, or at least all the examples we have of it come from Gondorian use.