To demist a car window is to clear the soft white cloud of tiny water that sits on the glass, usually by turning on the heat and the fan so the glass warms up. British speakers say demist; American speakers almost always say defog, or they talk about the defroster. In a British car you demist the windscreen so you can see the road. The mist comes from wet air inside the car meeting cold glass — it is not smoke, and it is not rain on the outside. It is an ordinary driving word in the UK, not a fancy one.

除霧（把車窗上的霧氣除掉。美式英語多說 defog）