Wikipedia
Sindarin is a fictional language devised by J. R. R. Tolkien for use in his secondary world known as Arda. 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).
Called in English "Grey-elvish" or "Grey-elven", it was the language of the Sindarin Elves of Beleriand. These were Elves of the Third Clan who remained behind in Beleriand after the Great Journey. Their language became estranged from that of their kin who sailed over sea. Sindarin derives from an earlier language called Common Telerin, which evolved from Common Eldarin, the tongue of the Eldar before their divisions, e.g., those Elves who decided to follow the Vala Oromë and undertook the Great March to Valinor. Even before that the Eldar Elves spoke the original speech of all Elves, or Primitive Quendian.
In the Third Age (the setting of The Lord of the Rings), Sindarin was the language most commonly spoken by most Elves in the Western part of Middle-earth. Sindarin is the language usually referred to as the elf-tongue or elven-tongue in The Lord of the Rings. When the Quenya-speaking Noldor returned to Middle-earth, they adopted the Sindarin language. Quenya and Sindarin were related, with many cognate words but differing greatly in grammar and structure. Sindarin is said to be more changeful than Quenya, and there were during the First Age a number of regional dialects. The tongue used in Doriath (home of Thingol King of the Sindar), known as Doriathrin, was said by many Grey-elves to be the highest and most noble form of the language.
In the Second Age, many Men of the island of Númenor spoke Sindarin fluently. Their descendants the Dúnedain of Gondor and Arnor continued to speak Sindarin in the Third Age. Within this fictional universe, Sindarin was first written using the cirth, an Elvish alphabet. Later, it was usually written in tengwar. Tolkien based the sound and some of the grammar of Sindarin on Welsh, and Sindarin displays some of the consonant mutations that characterize the Celtic languages. The language was also influenced by Old English and Old Norse.