Sindarin is Tolkien’s Elvish language associated with the Grey-elves of Middle-earth.
Tolkien based much of Sindarin’s sound and structure on Welsh, one of the languages he admired. In his fictional history, Sindarin developed among the Sindar, or Grey-elves, who remained in Middle-earth rather than completing the journey to Aman. It became a major spoken language in the northwestern parts of Middle-earth.
Sindarin is often confused with Quenya. Quenya is another major Elvish language, inspired partly by Finnish and associated with the High Elves and formal or literary use. Tolkien created related forms, sound changes, and historical stages for both languages, making them more than simple lists of invented words.