A diagram is a two-dimensional geometric symbolic representation of information according to some visualization technique. Sometimes, the technique uses a three-dimensional visualization which is then projected onto the two-dimensional surface. The word graph is sometimes used as a synonym for diagram.
The term diagram in common sense can have a general or specific meaning:
- visual information device : Like the term "illustration" the diagram is used as a collective term standing for the whole class of technical genres, including graphs, technical drawings and tables.
- specific kind of visual display : This is only the genre, that show qualitative data with shapes that are connected by lines, arrows, or other visual links.
In science the term is used in both ways. For example Anderson (1997) stated more generally: "diagrams are pictorial, yet abstract, representations of information, and maps, line graphs, bar charts, engineering blueprints, and architects' sketches are all examples of diagrams, whereas photographs and video are not". On the other hand Lowe (1993) defined diagrams as specifically "abstract graphic portrayals of the subject matter they represent".
In the specific sense diagrams and charts contrast computer graphics, technical illustrations, infographics, maps, and technical drawings, by showing "abstract rather than literal representations of information". The essences of a diagram can be seen as:
- a form of visual formatting devices
- a display that do not show quantitative data, but rather relationships and abstract information
- with building blocks such as geometrical shapes connected by lines, arrows, or other visual links.
Or in Hall's (1996) words "diagrams are simplified figures, caricatures in a way, intended to convey essential meaning". These simplified figures are often based on set of rules. The basic shape according to White (1984) can be characterized in terms of "elegance, clarity, ease, pattern, simplicity, and validity". The elegance for a start is determined by whether or not the diagram is "the simplest and most fitting solution to a problem".
Main diagram types 
There are at least the following types of diagrams:
- Graph-based diagrams: these take a collection of items and relationships between them, and express them by giving each item a 2D position, while the relationships are expressed as connections between the items or overlaps between the items; examples of such techniques:
- Chart-like diagram techniques, which display a relationship between two variables that take either discrete or a continuous ranges of values; examples:
- Other types of diagrams, e.g.,
Many of these types of diagrams are commonly generated using diagramming software. Thousands of diagram techniques exist. Some more examples follow.
Specific diagram types 
See also 
|Wikimedia Commons has media related to: diagrams|
|Look up diagram in Wiktionary, the free dictionary.|
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- Diagramming software
- Diagrammatic reasoning
- List of graphical methods
- Mathematical diagram
- Plot (graphics)
- commons:Specific diagram types – gallery of many diagram types.
- commons:Commons:Diagram resources
- Brasseur, Lee E. (2003). Visualizing technical information: a cultural critique. Amityville, N.Y: Baywood Pub. ISBN 0-89503-240-6.
- Michael Anderson (1997). "Introduction to Diagrammatic Reasoning". Retrieved 21 July 2008.
- Lowe, Richard K. (1993). "Diagrammatic information: techniques for exploring its mental representation and processing". Information Design Journal 7 (1): 3–18. doi:10.1075/idj.7.1.01low.
- Bert S. Hall (1996). "The Didactic and the Elegant: Some Thoughts on Scientific and Technological Illustrations in the Middle Ages and Renaissance". in: B. Braigie (ed.) Picturing knowledge: historical and philosophical problems concerning the use of art in science. Toronto: University of Toronto Press. p.9
- White, Jan V. (1984). Using charts and graphs: 1000 ideas for visual persuasion. New York: Bowker. ISBN 0-8352-1894-5.
Further reading 
- Bounford, Trevor (2000). Digital diagrams. New York: Watson-Guptill Publications. ISBN 978-0-8230-1572-6.
- Michael Anderson, Peter Cheng, Volker Haarslev (Eds.) (2000). Theory and Application of Diagrams: First International Conference, Diagrams 2000. Edinburgh, Scotland, UK, September 1–3, 2000. Proceedings.
Garcia, M (Ed) (2012) The Diagrams of Architecture. Wiley. Chichester.