The bootlid tailgate is the lifting cover for the boot space on a saloon vehicle or the lifting door on a hatchback or estate vehicle. In the latter it will include the rear window, referred to as the bootlid tailgate hatch glass, in the former it will be a solid piece of bodywork. 

The bootlid is attached by hinges to the boot space/luggage compartment at the rear of the vehicle. The bootlid tailgate hatch glass is set into the hinged bootlid tailgate, forming a window, which lifts, opening the rear end of the passenger compartment. Glass used in modern car doors is generally safety glass which decreases the chance of injury if it smashes. 

The bootlid tailgate hatch glass can, in some models, house a heated screen with a wiper/wash system and possibly lighting.  All of the windows in the car serve a safety function, offering protection to the people travelling in the car. All windows should be clean and free from cracks and scratches so that safety is not compromised.

When replacing windows, such as the bootlid tailgate hatch glass, it's important to make sure the part is compatible with the make and model of your car. The usual reason for replacement of the bootlid tailgate hatch glass is collision damage.