Sindarin is a fictional language devised by J. R. R. Tolkien for use in his secondary world known as Middle-earth. Sindarin is one of the many languages spoken by the immortal Elves, called the Eledhrim or Edhellim in Sindarin. The word Sindarin is itself a Quenya form. The only known Sindarin word for this language is Eglathrin, a word probably only used in the First Age (see Eglath).
Dictionary source: Tolkien´s Glossary
English to English translation of Sindarin
Of or pertaining to the Sindar. The Elvish tongue of Beleriand, derived from the common Elvish speech but greatly changed through long ages from Quenya of Valinor; acquired by the Noldorin exiles in Beleriand. Called also the Grey-elven tongue, the tongue of the Elves of Beleriand, etc. The tongue of the Grey-elves. The language of the Grey-elves of Beleriand and most commonly used of the Elf-tongues in Middle-earth. When the Noldor returned to Middle-earth at the beginning of the First Age, Thingol decreed that their tongue, Quenya, was not to be spoken in his realms. So the language of his people, Sindarin, came to dominate. Quenya, though, was still used as a language of ceremony and poetry. Sindarin shared common roots with Quenya, and the two languages had many similar words. Sindarin was said to be more changeful than the older tongue, however, and there were a number of regional 'dialects' of the tongue. The Sindarin spoken in Doriath was said to be the highest and most noble form of the language. The word Sindarin means '(Language) of the Grey People'.