Unified Modeling Language (UML) is a visual modeling language used in software development to create diagrams that represent various aspects of a system. It helps software developers and analysts to communicate and understand the various functionalities of a system. UML consists of a set of standardized diagrams that provide a visual representation of the system's architecture, design, and behavior. Some examples of UML diagrams include activity diagrams, class diagrams, and sequence diagrams. Understanding UML is essential for software professionals who work on designing, developing, and testing software systems.