What is a DFD$?$

Domain Flow Diagram, a model to formally specify the domain model, knowledge and concepts related to the system to be implemented

Data Flow Diagram, a set of diagrams to depict input and output data, as well as their relationships and associations

Direct Flow Diagram, a set of models to formally specify a domain of knowledge and related concepts

Data Fusion Diagram, graphic representations to model the flow of information in a given system