Sindarin is the Elvish language Tolkien created for the elves of Beleriand. In Tolkien’s mythology, it developed from an earlier Elvish language and became the commonly spoken Elvish tongue in much of Middle-earth during the First and later Ages.
Tolkien designed Sindarin as part of a connected family of invented languages rather than as a collection of isolated names. Its sound and structure were influenced especially by Welsh, while Quenya, another major Elvish language, drew more strongly on Finnish and Latin features.
Sindarin is spoken by characters including Legolas and many Elves of Middle-earth. Quenya remained a prestigious, ancient language associated with the High Elves, so the two names are often confused. They are related within Tolkien’s fictional linguistic history but are not interchangeable.
Tolkien’s language creation began decades before The Lord of the Rings was published. His linguistic invention was central to the development of the peoples, histories, and place-names of his legendarium.