XML Schema Definition (XSD) is a language used to define the structure and rules of an XML document. It specifies the elements, attributes, and data types that can appear in the document, ensuring that the data adheres to a predefined format. This helps in validating the content of the XML, making it easier to share and process data between different systems. By using XSD, developers can create a clear blueprint for their XML documents, which enhances data integrity and consistency. It also allows for better documentation and understanding of the data structure, facilitating easier maintenance and updates over time.