Sindarin was J.R.R. Tolkien’s Elvish language influenced by Welsh.
Sindarin is one of the major Elvish languages of Middle-earth. Tolkien based much of its sound and structure on Welsh, whose phonology and appearance he admired. In the fictional history, Sindarin developed from an older Elvish language and became the everyday tongue of many Elves in Beleriand and later in Middle-earth.
Quenya is the other best-known Elvish language, but Tolkien associated it more strongly with Finnish and Latin. Quenya functioned as a formal or ancient language among many Elves, while Sindarin was widely spoken in the narratives of The Lord of the Rings.
Tolkien developed both languages as part of a broader network of related tongues rather than inventing isolated vocabulary lists. His linguistic interests came before, and helped generate, many parts of his mythology. The languages therefore have internal histories, sound changes, and dialectal relationships.