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In the context of biological membranes, the reversal potential of a particular ion species is the membrane voltage at which there is no net flow of ions from one side of the membrane to the other. The potential can be calculated from the nernst equation. Ion channels conduct most of the flow of simple ions in and out of cells.
When a channel type that is selective to one species of ion dominates within the membrane of a cell--because other ion channels are closed, for example--then the voltage inside the cell will equilibrate to the reversal potential for that ion (i.e. assuming the outside of the cell is at 0 volts). For example, the "resting potential" of most cells is close to the potassium reversal potential. During a stereotypical action potential, the small resting conductance mediated by potassium channels is overwhelmed by the opening of a large number of sodium channels, which brings the membrane potential close to the reversal potential of sodium. When a cell has significant permeabilities to more than one ion, the cell potential can be calculated from the Goldman-Hodgkin-Katz equation.
More at: electrochemical potential -- cell potential
