A block diagram is a visual representation of a system that uses simple, labeled blocks that represent single or multiple items, entities or concepts, connected by lines to show relationships between them. An entity relationship diagram (ERD), one example of a block diagram, represents an information system by showing the relationships between people, objects, places, concepts or events within that system. (See an image on the ERD definition page.)

Block diagrams are used heavily in engineering and design of diagrams for electronics, hardware, software and processes. Most commonly, they represent concepts and systems in a higher level, less detailed overview. The diagrams are useful for troubleshooting technical issues. 

Block diagrams are a generalized representation of a concept and are not intended to display complete information in regards to design or manufacture. Unlike schematics, blueprints and layout diagrams, block diagrams do not portray the necessary detail for physical construction. Block diagrams are made simple so as not to cloud concepts. 

The simplification in block diagrams can also be useful when demonstrating an idea, but concealing the inner workings of potentially secret intellectual property (IP). Top-down design in electrical engineering often progresses through increasingly detailed block diagrams. After enough detail is added through iterations, the block diagram becomes a schematic. Block diagrams in process control show the functions of operations but not the components that perform them. The functions of block diagrams may then be implemented with programmable logic controllers (PLC).

Block diagrams are also used in a scientific context. In the study of biology, for example, block diagrams are used to display biological functions and interrelations.