The resting membrane potential of a cell is a voltage which can be measured across the membrane of a living cell. Inactive nerve and muscle cells measure between -50 and -100mV relative to the extracellular environment. This difference is due to differences in the distribution of ions within the cell relative to the extracellular environment.
Source: Despopoulos, A., Sibernagl, S. (2003). Color Atlas of Physiology (p. 44). New York: Thieme.