UML

UML (Unified Modeling Language) is a standardized, general-purpose visual modeling language maintained by the Object Management Group for specifying, visualizing, and documenting the structure and behavior of software systems. It grew out of the object-oriented methods of the 1990s into a single notation that development teams could share, and it defines a whole family of diagram types rather than one drawing style. Its purpose is to give people a common, tool-independent way to talk about a system’s design.

• Structural diagrams - Class, component, and deployment diagrams describe how a system is put together.
• Behavioral diagrams - Sequence, activity, and state diagrams describe how a system acts over time.
• OMG standardization - A formal specification from the Object Management Group keeps the notation consistent across tools and vendors.
• Tool-independent notation - The same diagrams can be drawn, exchanged, and reasoned about in many different modeling tools.

In real API work UML most often shows up in sequence diagrams that map a call flow across services and in class or component diagrams that inform a data model before it becomes an OpenAPI or JSON Schema contract. It is a design and communication artifact more than a machine-executable one — teams reach for it to reason about a system with stakeholders, then translate that understanding into the concrete API specifications and code that governance actually enforces. Where lighter text-based diagramming has taken over much day-to-day sketching, UML remains the shared vocabulary behind how many organizations still document architecture.