Unbalanced circuit

It is usually taken to mean that one pole of each port is bonded to a common potential (single-ended signalling) but more complex topologies are possible.

This scheme is typical of many electronic circuits that are not required to have differential inputs or outputs.

The difficulties come when a port of the circuit is to be connected to a transmission line or to an external device that's designed for balanced operation.

Many transmission lines are intrinsically an unbalanced format such as the widely used coaxial cable.

If that is not possible or desirable, a balun, a device for interfacing balanced and unbalanced circuits, may be used.