The term diagram in its commonly used 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 the genre that shows 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, engineeringblueprints, 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".
a display that does not show quantitative data (numerical 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 a set of rules. The basic shape according to White (1984) can be characterized in terms of "elegance, clarity, ease, pattern, simplicity, and validity". Elegance is basically 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:
^Lowe, Richard K. (1993). "Diagrammatic information: techniques for exploring its mental representation and processing". Information Design Journal7 (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. ISBN0-8352-1894-5.
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.