"NoSQL" refers to a category of data storage solutions designed to handle unstructured and semi-structured data that traditional relational databases may struggle with. These systems prioritize flexibility, scalability, and performance, often using a variety of models, such as document, key-value, or graph databases. NoSQL databases allow for dynamic schema design, enabling rapid development and iteration of applications. They are particularly suitable for big data applications, real-time analytics, and managing large volumes of data across distributed systems. Popular NoSQL databases include MongoDB and Cassandra.