A plan, sketch, drawing, or outline designed to demonstrate or explain how something works or to clarify the relationship between the parts of a whole.
Mathematics. A graphic representation of an algebraic or geometric relationship.
A chart or graph. [DICT1]
For our purposes in this book, a diagram is the layout and visualization of different modeling element0s as described within the UML. Each UML diagram is used for a specific purpose, typically to visualize a certain aspect of your system (summarized next). Each diagram uses specific UML symbols to achieve its purpose.
The UML contains two different basic diagram types: structure diagrams and behavior diagrams. Structure diagrams depict ...