Definition Related to DFD

     A Data Flow Diagram is a network model of an information processing system. The arcs of the network represent data flows, and the nodes represent data stores, transforms, or selected elements of the environment.

A Data Flow is a movement of information within the system or across the system boundary.

A data flow which crosses the system boundary to enter the system is called a net system input  or simply System Input. 

A data flow which crosses the boundary to leave the system is called a net system output or System Output.

A Data Store is a time-delayed repository of information, where data is kept temporarily or permanently, avoid thinking of them in computer "file" terms, there are many quite different forms.

A Process, or Transform, is a transformation that inputs one type of data and outputs a different type. It is a conversion of data from one form to another. A process cannot output data without getting some in, data cannot be created in a process.

To show the connection between a system and its environment, a fourth type of element is used. 

A Source (origin) is a person or organisation or system outside the system under consideration that provides information to the system under study in the form of an incoming data flow. 

A Sink (destination) is a person or organisation or system outside the system under study that receives a system output. 

Sources and sinks are sometimes collectively called External Entities. These are often the users, suppliers of commodities, or customers of the system.