Sindarin is the fictional Elvish language associated with the elves of Beleriand and western Middle-earth.
Tolkien developed Sindarin from the language forms used by the Sindar, or Grey-elves. It is one of the most prominent Elvish languages in The Lord of the Rings, appearing in names such as Galadriel, Celeborn, Rivendell, and Minas Tirith. Its sound patterns were influenced chiefly by Welsh.
Quenya is the other major Elvish language in Tolkien’s mythology, but it is associated especially with the High Elves and has a more classical or formal role. Sindarin and Quenya are therefore not interchangeable names for one language, even though both belong to Tolkien’s Elvish linguistic history.