XML, or Extensible Markup Language, is a markup language used to encode and store data in a structured and machine-readable format. XML is designed to be both human-readable and machine-readable, allowing developers to create custom tags and data structures that can be easily parsed and processed by software applications. XML documents are hierarchical in nature and can include attributes, text and other data types. It is widely used in web development, software development, data exchange and other fields where structured data is required.