A NoSQL database is a type of database that provides a mechanism for storage and retrieval of data that is modeled in ways other than the tabular relations used in traditional relational databases. NoSQL databases are designed to handle unstructured or semi-structured data, making them suitable for applications that require flexible data models, high scalability, and fast access to large volumes of data. Unlike relational databases, which use SQL for querying, NoSQL databases support various data models, including document, key-value, column-family, and graph formats.

NoSQL databases are particularly well-suited for modern applications such as big data analytics, real-time web applications, and Internet of Things (IoT) systems. They offer advantages like horizontal scalability, which allows them to efficiently distribute data across multiple servers, and schema flexibility, enabling developers to adapt the data structure as requirements evolve. However, NoSQL databases may trade off some of the consistency guarantees provided by relational databases in favor of availability and partition tolerance, a trade-off often referred to in the context of the CAP theorem.