Sindarin is the fictional language created by J.R.R. Tolkien for the Sindar, or Grey-elves.
Within Tolkien’s legendarium, Sindarin developed from an earlier Elvish language and was associated especially with the Sindar of Middle-earth. It became a widely used Elvish language in the later histories of the First and Second Ages and remained prominent in the Third Age. Many names in The Lord of the Rings, including Rivendell’s Elvish name Imladris, reflect Tolkien’s use of Sindarin.
Quenya is the other best-known Elvish language. Tolkien associated it particularly with the High Elves and modeled aspects of its sound and structure on Finnish. Sindarin, by contrast, was influenced in part by Welsh phonology. Both languages are distinct, developed systems rather than random collections of invented words.
Khuzdul is the language of the Dwarves, while the Black Speech was created for the servants of Sauron. These languages are therefore not interchangeable with Sindarin, even though characters from different peoples encounter one another throughout Tolkien’s stories.