Final answer:
Two Data Flow Diagrams (DFDs) with equivalent external data flows are described as isomorphic, meaning they have a one-to-one correspondence between their data flows.
Step-by-step explanation:
When 2 Data Flow Diagrams (DFDs) have equivalent external data flows, we can describe the DFDs as being isomorphic. An isomorphic relationship in this context means that the two DFDs have data flows that can be mapped to each other, one-to-one, maintaining the structure and information flow of the system. Each data flow in one DFD has a corresponding data flow in the other DFD with the same input and output criteria, although the processes and data stores within the system might be arranged differently. This concept is important in systems analysis and design when comparing the data handling of two system representations to ensure consistency and completeness.