A flow diagram, or flowchart, is a specific type of activity diagram that communicates a sequence of actions or movements within a complex system. A flow diagram is a powerful tool for optimizing the paths of people, objects, or information.

The term flow diagram is also used as a synonym for flowchart, and sometimes as a counterpart of the flowchart. Flow diagrams are used to structure and order a complex system, or to reveal the underlying structure of the elements and their interaction.