What is Membrane potential?

Every living cell holds an electrical potential difference across its plasma membrane, the membrane potential $V_m = V_{\text{in}} - V_{\text{out}}$, negative inside: $-70\,\mathrm{mV}$ in a neuron, $-90\,\mathrm{mV}$ in a muscle fibre, $-120\,\mathrm{mV}$ or more in a plant cell. It arises because the membrane is selectively permeable to ions whose concentrations differ across it.