What is ozone depletion? What causes it? Is it reversible?
It is a relative change in the amount of ozone in the stratosphere.
It is caused when the ozone normally created by reaction of atmospheric oxygen with cosmic rays is consumed faster than it can be replaced. It is consumed by reactions with chlorofluorocarbons (NOT "greenhouse gases"). It is replenished naturally when those pollutants are not present.
The Montreal Protocol agreement banned the use of those chemicals, and the reduction of use has helped the ozone concentration to increase again. Maps, history, and more details can be found here: