RELAX NG (Regular Language for XML Next Generation) is a schema language for XML. The .relax extension specifically refers to files written in the compact syntax of RELAX NG. These files define the structure and content rules for XML documents. They specify which elements and attributes are allowed, their order, and the data types they can contain. RELAX NG is known for its simplicity, expressiveness, and modularity, making it a popular choice for validating XML data. The compact syntax is a human-readable alternative to the XML syntax of RELAX NG, making it easier to write and maintain schemas. RELAX NG schemas are used to ensure that XML documents conform to a specific format, which is crucial for data exchange, validation, and processing in various applications. They are often used in web services, data integration, and document management systems to enforce data quality and consistency.