J.R.R. Tolkien called the fictional invention of languages glossopoeia.
The term comes from Greek roots meaning roughly “tongue” and “making.” Tolkien treated language invention as a creative art rather than as a list of arbitrary words. He developed sound systems, grammar, vocabulary, scripts, and historical relationships for languages such as Quenya and Sindarin, giving them the sense of languages that had changed over time.
Glossopoeia is related to but distinct from mythopoeia, the creation of myths and imagined worlds. Tolkien’s languages helped generate stories and cultures, while his stories also gave the languages fictional historical settings. His method differed from a simple coded language because he created both internal history and stylistic variation. Quenya drew inspiration partly from Finnish, while Sindarin was influenced in part by Welsh. These influences did not make the languages identical to their sources.