What tool is used to define boundaries for business domains?

The chosen answer refers to a diagram, which serves as an essential tool in visualizing and defining boundaries for business domains. Diagrams, in general, provide a graphical representation that can convey complex concepts and relationships in a simplified manner. They help stakeholders understand and agree on the scope of the business domain, facilitating discussions and clarifying requirements.

While entity relationship diagrams and UML diagrams are specific types of diagrams that model relationships and system behavior respectively, they are often used for more detailed modeling, such as showing data relationships or system components. A data flow diagram is particularly focused on the data movement within systems, rather than outlining the business boundaries themselves. Hence, the use of a general term like "diagram" encompasses various forms which can be employed selectively to illustrate the domain's scope and context, making it the most versatile answer in this scenario.