Param is a zero-dependency library for declaring rich attributes - Parameters - with extended metadata used for various purposes such as runtime type and range validation, default values or factories, nullability, docstrings, etc. It also supplies a suite of reactive APIs (watch, depends, bind, and reactive expressions), enabling automatic updates on attribute changes and declaring complex reactive dependencies and expressions that can be introspected by other frameworks to implement their own reactive workflows. These capabilities power HoloViz projects like Panel and HoloViews, making Param a solid foundation for building validated, introspectable, and responsive libraries and applications.