Sindarin is the Tolkien language spoken by many Elves of Beleriand in The Silmarillion. Tolkien based much of its sound and structure on Welsh, one of the real-world languages that strongly influenced his linguistic imagination.
Sindarin is associated especially with the Sindar, or Grey-elves. It became the common Elvish language of much of Middle-earth during the later First Age and remained widely used in the Third Age. Names such as Galadriel, Celeborn, and Minas Tirith contain Sindarin elements.
Quenya is another major Elvish language, modeled in part on Finnish and treated as a more formal or ancient tongue in many settings. The two languages are therefore related within Tolkien’s mythology but are not interchangeable.
Tolkien developed Sindarin across many drafts and versions of his stories. Its vocabulary and grammar were never published as a complete ordinary-language course, although fans have reconstructed substantial linguistic information from his writings.