Shunt is a general term meaning to divert flow of current. For the purpose of battery monitoring, a shunt is a very low resistance resistor in series with the load. By measuring the voltage drop across the resistor, the current into the load (or into or out of the battery) can be measured.
Here is a discussion of how it works:
Battery shunts for current monitori – what They are & why you need one -
Electrical shunts explained. 100 Amp/100mV, 500Amp/50mV Ohm's Law battery monitor Trimetric Victron BMV Renogy
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