Electrical measurement range is limited by the current which can be carried by the coil of the instrument safely. Hence for measurement of large current, voltage or power some means are used for increasing the range of instrument. Such mean is also called shunt.
The ammeter shunt is the device which provides the low resistance path to the flow of current. It connects in parallel with the ammeter. In some of the ammeter, the shunt is inbuilt inside the instrument while in others it is externally connected to the circuit.