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> Voltage, electric potential difference, electric pressure or electric tension is the difference in electric potential between two points, which (in a static electric field) is defined as the work needed per unit of charge to move a test charge between the two points. (from [Wikipedia](https://en.wikipedia.org/wiki/Voltage))
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What exactly is a point in this definition? The voltage of a cell can be measured for example and a cell is not exactly what I intuitively understand to be a point. Let's say I have such a cell with an electrical potential $V\_m$ of $-80mV$ (is this the same as saying the cell has a voltage of $-80mV$?).

Am I right then that the voltage is a relative measure between the cell and the extracellular fluid? And is the voltage of this fluid then just the symmetric inverse, i.e. $V\_{extra}=80mV$? How then can the voltage be measured between two bodies, because the concentration of charged ions might not be uniform across the fluid (or across the cell)? Could the voltage also be measured between two organelles within the cell or between the entire human body and the air surrounding it?