What is the exact definition of mitochondrial depolarization?
The inner mitochondrial membrane is important for ATP synthesis (oxidative phosphorylation) to occur. Electron transport chains are situated on the inner mitochondrial membrane where electrons traverse the chain across proteins - causing protons to traverse into the mitochondrial matrix. Creating an electrochemical proton gradient.
This is a very interesting topic and is best understood with an explanation of the JC-1 Dye.
Explanation
Mitochondrial membrane potential changes - depending on cellular events.
The dye 'JC-1' is able to indicate mitochondrial membrane potential in different cells.

If there is a negative charge inside mitochondria then the monomers express green fluorescence.
If there is a positive charge inside mitochondria then the monomers form J-aggregates and now express red fluorescence.

The ratio between red &amp; green florescence shows the mitochondrial membrane potential.

A low or negative mitochondrial membrane potential will show more green florescence.
A high or positive mitochondrial membrane potential will show more red florescence.

Example
One event that may influence mitochondrial membrane potential is an increase in reactive oxygen species.
Reactive oxygen species are molecules that damage cells but are also important to regulate cellular activities. Some examples are Hydrogen Peroxide or Superoxide.
In this image cells are stained with JC-1. The ratio of the dye is ~1:1 at basal. As the reactive oxygen species hydrogen peroxide is added and time continues, there is more green. Showing:

A low or negative mitochondrial membrane potential in each of the cells.