Question

what is potential difference?

Answer 1

electrostatic potential difference between two points is defined as the work done to bring a unit positive charge from one point to another in an electric field  without acceleration
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Its SI unit is volt 
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And its Si unit is defined as 

The potential difference is said to be 1V when 1j of wrk is done to bring a unit positive charge from one points to another without acceleration
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Answer 2

it is the difference of electrical potential between two points.

Voltage electric potential difference electric pressure or electric tension is the difference in electric potential energy between two points per unit charge