Depolarization

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 rising and falling phases of an action potential are often imprecisely called depolarization and hyperpolarization, respectively.

Vernacular usage
In the study of membrane potentials, particularly in the field of action potentials, depolarization has taken on an informal, technically incorrect meaning. For action potentials, a major concern is changes in membrane potential from the resting potential. For understanding the action potential, the resting potential is more important than the mathematically significant potential of zero. The resting potential, by convention, has a negative value. The initial depolarization during the rising phase of an action potential is in the direction from the negative resting potential towards the positive membrane potential that will be the peak of the action potential. In discussing action potentials, this direction of change in membrane potential (from negative to positive) is "the depolarizing direction from the resting potential". In discussing action potentials, this is often shortened to "the depolarizing direction", or, simply, "depolarization".

As described at the top of this page, depolarization is any change in membrane potential that changes the value of the membrane potential to be closer to zero. However, a membrane potential that grows from a small positive value to a more positive value (i.e. becoming more polarized) is technically hyperpolarizing, making the vernacular usage of the term depolarization incorrect for the part of the rising phase of the action potential when the value of the membrane potential is positive (0 millivolts to +40 millivolts in the figure). Despite it being technically incorrect, text books often use the vernacular definition of "depolarization" (membrane potential change in the depolarizing direction from the resting potential) in their descriptions of electrophysiology.

See Hyperpolarization for additional comparison of the correct and vernacular usage of the terms "depolarization" and "hyperpolarization".