Morben (pl. Morbin or Moerbin) was a term used amongst Sindar that lived in Beleriand (and Nandor). Avari or anybody else dwelling outside Beleriand or entering Beleriand from outside was called a Morben.
Morben is usually translated 'Dark-elves' (cf. Q. Moriquendi), since it was used about the elves (and people) who were hostile to Eldar and Edain. However, it was at once used of the Easterlings that appeared before the Battle of Nirnaeth.
In the same passage with Morben, the word Calben is introduced. Cf. this for usage.
Source: HoME XI, 362; 376-377; 380; 408-409.