To defog is to clear the white mist off glass, most often the windows inside a car, by using heat or the air system. You turn a button, warm air hits the glass, and the cloud goes so you can see. Drivers and car manuals use this word. You can also defog glasses or a bathroom mirror, but the car use is the main one. American English likes defog; British English often says demist for the same job. It is a practical everyday verb, not a fancy one. The noun you press is often called the defogger, or just “the defog.”