what is a context diagram

DFDs can be expressed as a series of levels. The outermost level (Level 0) is concerned with how the system interacts with the outside world. A context diagram can define as a graphical representation of a system which deliberately does not go into defining processes. So it is a data flow diagram with only one massive central process that subsumes everything inside the scope of the system. 

It is just to give an overview of a system without getting bogged down in detail. It is a top level data flow diagram. Context diagram also known as Level 0. In Level0 the diagram does not contain any Data stores..When drawing a context diagram there are three symbols used in a context diagram such as circle, arrows and a rectangle.

Circle               : Represent the system in terms of a single process,

Arrows            : Show data flow

Rectangle        : Show any external entities affecting the system.