YAML (a recursive abbreviation denoting “YAML Ain’t Markup Language”) bills itself as a human friendly data serialization standard for all programming languages. In my experience, it’s mostly popular in the Python ecosystem. It’s a relatively pleasant language for fairly straightforward configuration languages (Ansible, in particular), though its use in tools like Cloud Custodian leads one to lean heavily on some of the wacky side features in the YAML spec. Pleasantly, there’s a bijection between YAML and JSON structures, so there’s plenty of tooling allowing a user to work in their preferred language and convert for the benefit of picky programs. All told, though, I’d much prefer everyon just use Dhall since it’s stricly better (if a bit less ergonomic on day one).