Sindarin is the principal language of the Grey-elves in J.R.R. Tolkien’s Middle-earth.
Tolkien based much of Sindarin’s sound and historical development on Welsh, one of the real-world languages that influenced his invented-language work. In the stories, Sindarin was widely spoken by Elves in Middle-earth, including many who were not part of the high-elven communities associated with Quenya.
Quenya and Sindarin are related within Tolkien’s fictional history, but they have different roles and sound patterns. Quenya is associated especially with the High Elves and has a more formal or ceremonial status in many contexts. Sindarin became the everyday Elvish language of many communities in Middle-earth.
The word “Sindarin” itself is an English-language label used by Tolkien’s readers and editors; within the fictional linguistic history, the language has other names and forms. Its prominence is why many Elvish names in The Lord of the Rings have Sindarin origins.