What is UML? Explain what a diagram, notation, and metamodel are in UML.

UML (Unified Modeling Language) is a standard graphical modeling language for visualizing, specifying, designing, and documenting software systems.

A diagram in UML is a graphical representation of a system model or part of it. Diagrams help understand the structure and behavior of the system.

Notation in UML is a set of graphical elements (symbols and lines) and rules for their use in constructing diagrams. The notation is unified so that models can be understood by different developers.

A metamodel in UML is a model of a model. It defines the basic elements, their relationships, and rules that can be used to create any UML models. The metamodel ensures consistency and formal definition of the UML language.