Sindarin (the language of Sindar) was one of various languages invented by Tolkien and spoken in middle earth, a fictional fantasy world, best known from the Hobbit and Lord of the Rings.
There were two Eldarin languages spoken in the Lord of the Rings: Quenya (High-elven) and Sindarin (Grey-elven).
High-elven (Quenya) was an ancient tongue from Eldamar beyond the sea. It has been described as an Elven-latin, that is a ceremonial language and not used for everyday speech. It was spoken by the High Elves.
Sindarin is a described in the stories as a descendant of Quenya. In fact in addition to these two language Tolkien developed various constructed languages such as Goldogrin. Outside of the Middle Earth mythology, it could be said that Sindarin was descended from these original constructed languages.
Tolkien was also interested in languages in general, and the languages he contructed reflected these interests.
ae Adar nín i vi Menel
no aer i eneth lín
tolo i arnad lín
caro den i innas lin
bo Ceven sui vi Menel
anno ammen sír i mbas ilaurui vín
ar díheno ammen i úgerth vin
sui mín i gohenam di ai gerir úgerth ammen
As the primary use is for a crossword, endings, suffices, speculative words and so on have been removed, and the dictionary included in case you are having difficulties completing the Sindarin crossword ;)
The words are those used by Tolkien around the time of publication of The Lord of the Rings. In other words, it doesn't mix words from different periods and doesn't include vocabulary from other of Tolkiens languages.