Theoretically, a recognized member of the modern state system possessing formal sovereignty and occupied by a people who see themselves as a single, united nation. Most nations and states aspire to parlance, nation-state is used as a synonym for country or state.
In biology, depolarization is a decrease in the absolute value of a cell's membrane potential. Thus, changes in membrane voltage in which the membrane potential becomes less positive or less negative are both depolarizations.
The sudden but short-lived depolarization of a region of the plasma membrane during an action potential is caused by a sudden massive, but transient, influx of Na+ ions through opened voltage-gated Na+ channels in that region. At the resting membrane potential these voltage-gated channels are closed.