Depolarisation

When something is polarised its ends are oppositely charged. So, depolarisation is when the difference is balanced, the ends no longer have different charges.

For example in neural stimulation, a neurone can become depolarised when the cells resting membrane potential (-75 mV) becomes more positive (+40 mV). This occurs when the cell is stimulated via an action potential; the membrane contains voltage-gated sodium channels that become more permeable to sodium ions allowing them to diffuse out of the cell down their concentration gradient, making the overall charge more negative relative to the outside.