Final answer:
The symbol that represents a source or destination of data outside a system in data flow diagrams is an external entity or terminator, depicted as rectangles or squares. These denote entities outside the system that provide or receive data, with arrows showing the direction of data flow.
Step-by-step explanation:
In the context of data flow diagrams (DFDs), the symbol that portrays a source or destination of data outside a system is commonly referred to as an external entity or terminator. These are represented graphically as rectangles or squares. The purpose of these symbols is to show entities that interact with the system, but which are not part of the system itself. For instance, when drawing a DFD for a business, one might include rectangles to represent external entities such as customers, suppliers, or other outside systems that provide or receive data to or from the business's system.
An arrow from an external entity into the system represents inputs, such as data or materials coming into the system (imports), while arrows pointing outwards symbolize outputs, like products or information being sent from the system (exports). Through this use of standardized symbology in DFDs, one can clearly delineate the scope of a system and its interactions with external sources or destinations.