Sindarin is the fictional language J.R.R. Tolkien based mainly on Welsh.
Tolkien was a philologist with a strong interest in historical languages, and he designed Sindarin as one of the principal Elvish languages of Middle-earth. Its sound and structure were especially influenced by Welsh, while its history within the legendarium links it to earlier Elvish languages.
Sindarin became the everyday Elvish language of many Elves in Middle-earth, particularly the Sindar and other populations in the west. Names such as Lothlórien, Mithrandir, and Minas Tirith reflect Tolkien's linguistic construction and naming systems.
Quenya, another major Elvish language, was influenced chiefly by Finnish and has a more formal or ancient role in many contexts. The two languages are related within Tolkien's invented history but are not interchangeable.