(Also called second hand smoking)
"Passive smoking" means that the passive smoker gets all of the healthy and definitely less healthy chemicals of tobacco somehow, without actually actively inhaling tobacco smoke, by smoking cigarettes or such.
A passive smoker is someone who, for example, lives or works in the same room with someone who smokes there. Due to the wonder of air circulation, tobacco smoke ends up to the lungs of the non-smokers.
(I'm not saying anything about the health effects, just, as a non-smoker, mention that the smoke is Slightly Annoying. Ick, the smell...)
(Nyes, we rescue nodeshells.)