J.R.R. Tolkien used Eldar for the Elves who undertook the Great Journey toward the West.
In Tolkien’s languages and mythology, the Eldar were not simply every Elf in existence. The term originally referred to Elves who accepted the summons of the Valar and began the journey from Cuiviénen toward the western lands. Those who refused were called Avari.
The journey divided the Eldar into important groups, including the Vanyar, Noldor, and Teleri. Some reached Aman, while others remained in Middle-earth or returned there later. This history is central to The Silmarillion and to the conflicts surrounding the Silmarils.
A common mistake is treating Eldar as an alternative name for all Elves. Tolkien’s classification is more specific: it concerns the Elves associated with the Great Journey, even though the word can be used more broadly in some contexts.