Sindarin is the Elvish language J.R.R. Tolkien associated with the Grey Elves. In Tolkien’s mythology, it developed from the language of the Sindar, Elves who remained in Middle-earth rather than completing the journey to Aman.
Tolkien based much of Sindarin’s sound and historical development on Welsh, one of several real-world languages that influenced his invented languages. Sindarin is related to Quenya, but the two languages occupy different cultural and historical roles in Tolkien’s legendarium.
Quenya is associated especially with the High Elves and has a sound system influenced in part by Finnish. Sindarin became the more commonly spoken Elvish language in much of Middle-earth during the Third Age, which is why it appears frequently in names and dialogue connected with The Lord of the Rings.
The languages are not merely collections of invented words. Tolkien designed grammar, vocabulary, sound changes, and historical relationships, treating language creation as a central part of his world-building.