Eldar
The name Eldar refers to different races of Elves or Elf-like beings.
Eldar in Tolkien's legendarium
In the legendarium of J. R. R. Tolkien, the Eldar (Q. 'Star-people') are those Elves who answered the invitation of the Vala Orome to relocate to the Undying Lands: the Vanyar, the Noldor and the Teleri. This includes the Sindar and the Nandor, who did not complete the journey. The narrower term Calaquendi (Q. 'Light-elves') in contrast only refers to those Elves who actually went to Aman. Those Elves who did not take part in the Great March at all are called Avari (Q. 'Refusers').
Eldar in Warhammer 40,000
In the science fiction game Warhammer 40,000, the Eldar are an alien starfaring species. They are humanoid in appearance, with high skulls, long pointed ears, and long limbs; their inner anatomy and physiology, however, is very different from humans. The Eldar once ruled the Galaxy but their old empire has fallen in ruin long ago. Still, the Eldar possess highly advanced technology, with a preference for streamlined, elegant designs, and strong psychic powers.
The Dark Eldar are a faction that has split off the other Eldar.