What is the Elvish language called Sindarin in J.R.R. Tolkien’s legendarium?

In J.R.R. Tolkien’s legendarium, Sindarin is the language of the Grey Elves.

Sindarin developed from Tolkien’s broader Elvish language system and is associated especially with the Sindar, or Grey Elves, of Middle-earth. It is one of the best-known Elvish languages in Tolkien’s writings and is used by characters and place names throughout The Lord of the Rings.

Tolkien designed Sindarin with a historical relationship to other Elvish tongues. Its sound and development were influenced in part by Welsh, while Quenya, another major Elvish language, was influenced in part by Finnish and Latin. These languages are related within Tolkien’s fictional history, but they are not interchangeable.

A frequent mistake is to call every Elvish word “Quenya.” Quenya is strongly linked to the High Elves, whereas Sindarin became the everyday Elvish language of many Elves in Middle-earth during the later histories.