The nyaml converter is a Python software tool designed to transform the NeXus Definition Language (NXDL) from YAML format to XML, the official format for NeXus data exchange format (https://www.nexusformat.org/), and vice versa. NXDL is used to define experiment instruments, setups, or scientific data objects from experiments in a schema-based structure. While the official NXDL format is in XML, this tool enables users to write the schema in a more developer-friendly YAML format and seamlessly convert it into the official XML format. The `nyaml` tool also supports the reverse process, converting XML-based NXDL schemas into YAML format. Users can benefit from a more accessible and human-readable way to define NeXus data structures, while still adhering to NeXus standards. The tool requires users to follow a specific grammar (see full documentation at https://fairmat-nfdi.github.io/nyaml/) for embedding NeXus data structures, ensuring compatibility with the XML-based NXDL. This work is part of the FAIRmat project (project ID 460197019), funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) (https://gepris.dfg.de/gepris/projekt/460197019?language=en). Visit the [FAIRmat page](https://www.fairmat-nfdi.eu/fairmat/about-fairmat/consortium-fairmat) for more info.

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