A Dark Elf
In Middle-earth the term Dark Elves referred to the Umanyar, those Eldar who had never seen the light of Aman (the peoples of the Sindar and Nandor) and the Avari, the East-Elves or wild Elves.
The Sindar were also often considered neither Light Elves nor Dark Elves but Elves of Twilight.
Though the original schism had happened before Sun and Moon had been revealed being Dark Elf or Elda was essentially a matter of personal choice.An Avar who found the Grey Havens was still technically allowed to decide to finally make the Journey and live in the immortal lands or an Elda or one of their descendants could still decide to remain on Ambar, though of course the Two Trees of Valinor were already destroyed, thus they could not experience the direct exposure to their holy light anymore. In contrast to the High Elves, the Avari embraced the fading and becoming more houseless and less corporeal in time and saw it as a natural process if being part of the matter of Arda.
The Dark Elves should not be confused with Fallen Elves.