1) In the
work of JRR Tolkien, this is an elvish word used to refer to elves. More specifically, it refers to elves who made the journey west in the First Age, and their descendants (as opposed to the "dark elves" or Avari). All the major elven characters in Tolkien's novels are eldar.
2) In the Warhammer
40,000 (Games Workshop)universe, the Eldar are a race of
aliens who live on Craft-worlds. Broadly good in alignment, they are divided into different units of bizarrely coloured guilds and crafts.
Basically, futuristic elves. They dress in
slim, streamlined armour with pointed helmets, and have many special units relying on otherworldly technologies and magic - including some which look remarkably
like Star Wars speeder bikes, others which resemble floating disks, and giant godlike beings known as Avatars.