Any path along which electrons can flow is a circuit. For a continuous flow of electrons, there must be a complete circuit without any gaps. An example of a gap is an electric switch such as a light switch that can be closed to allow electron flow or open to cut it off.
There is a standard set of symbols used to represent the various parts of a circuit. In circuit diagrams a battery is shown as two parallel lines, one longer than the other. The longer line is the positive terminal and the shorter line is the negative terminal.