NoSQL is a term used to describe a class of non-relational database management systems that are designed for large-scale, distributed data storage and retrieval. Unlike traditional relational databases, which use a fixed schema and use SQL for data access, NoSQL databases are schema-less and use more flexible query languages, such as MapReduce, to allow for more agile development and real-time data processing. NoSQL databases are often used for big data and real-time web applications.