What are the .x files in /usr/include?

My /usr/include contains several files with the .x file extension, such as /usr/include/rpcsvc/rquota.x. They seem like C sources (running file /usr/include/rpcsvc/rquota.x results in C source, ASCII text), but they are not valid C (e.g. program and version seem keywords). What are they exactly? It's hard to google given the short extension, and some websites are just wrong/incomplete (e.g. [Wikipedia](https://en.wikipedia.org/wiki/.x) says "old DirectX files").