The term 'Critical Layer' refers to the layer of the atmosphere known as the OZONE layer. The OZONE layer is a region of the Earth's atmosphere that contains a high concentration of OZONE, which is a form of oxygen. This layer is important because it absorbs harmful ultraviolet radiation from the sun, protecting life on Earth from its damaging effects. The OZONE layer is also referred to as the 'OZONE shield' or the 'OZONE umbrella'.