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.
Eldar sounds a bit like a cross between "elder" and "elf".
Space elves. From WH40K.
The Eldar.. They refer to humans as the 'Mon-Keigh'.