In biology, depolarization is a change in a cell's membrane potential

Membrane potential

Membrane potential is the difference in electrical potential between the interior and exterior of a biological cell. All animal cells are surrounded by a plasma membrane composed of a lipid bilayer with a variety of types of proteins embedded in it...

, making it more positive, or less negative. In neurons and some other cells, a large enough depolarization may result in an action potential

Action potential

In physiology, an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and...

. Hyperpolarization

Hyperpolarization (biology)

Hyperpolarization is a change in a cell's membrane potential that makes it more negative. It is the opposite of a depolarization.Hyperpolarization is often caused by efflux of K+ through K+ channels, or influx of Cl– through Cl– channels. On the other hand, influx of cations, e.g...

 is the opposite of depolarization, and inhibits the rise of an action potential.
If, for example, a cell has a resting potential

Resting potential

The relatively static membrane potential of quiescent cells is called the resting membrane potential , as opposed to the specific dynamic electrochemical phenomena called action potential and graded membrane potential....

 of -70mV, once the membrane potential changes to -50mV, then the cell has been depolarized.