Sindarin is the principal Elvish language of Beleriand in J.R.R. Tolkien’s legendarium.
In Tolkien’s fictional history, Sindarin developed among the Sindar, or Grey Elves, who remained in Middle-earth rather than completing the journey to Valinor. During the First Age, it became the everyday language of many Elves in Beleriand, the western region that was later destroyed in the War of Wrath.
Tolkien modeled Sindarin’s sound system largely on Welsh, while Quenya was influenced more strongly by Finnish and other classical or historical languages. This distinction is useful because both languages are Elvish, but they occupy different cultural and historical roles in the legendarium.
Sindarin also appears in later ages, including The Lord of the Rings, where it is used by characters such as Elrond, Galadriel, and Legolas. Tolkien developed related forms and historical stages rather than treating the language as a static vocabulary list.