NHibernate is an open-source object-relational mapping (ORM) framework for the .NET platform. It allows developers to interact with databases using .NET objects, simplifying data manipulation and retrieval. By mapping database tables to .NET classes, NHibernate helps reduce the amount of boilerplate code needed for database operations. One of the key features of NHibernate is its ability to manage complex data relationships and transactions. It supports various database systems and provides a powerful query language called HQL (Hibernate Query Language). This flexibility makes NHibernate a popular choice for developers looking to streamline their data access layer.