Sindarin is the Elvish language J.R.R. Tolkien developed for the Elves of western Middle-earth.
In Tolkien’s linguistic history, Sindarin descends from an earlier language associated with the Teleri, while its speakers in Middle-earth include the Sindar, or Grey Elves. Its sound patterns were influenced partly by Welsh, one of the real-world languages Tolkien admired.
Sindarin appears in names and phrases throughout The Lord of the Rings. Examples include Lothlórien, Minas Tirith, and the name Galadriel. Tolkien created related languages and historical stages rather than treating Elvish as a single language.
The most important comparison is Quenya. Quenya is another major Elvish language, associated especially with the High Elves and influenced in sound by Finnish. Both are genuine Tolkien languages, so identifying a specific word or name requires attention to its fictional history.