Preventable describes something you can stop from happening — or stop someone from doing — if you act in time. A preventable accident is one that did not have to happen. Doctors talk about preventable sickness: the kind you can keep away with a vaccine, clean water, or a change of habit. The word is an adjective. It does not mean you already stopped the thing. It means stopping it was (or still is) possible. People often say it with a little heat: if a death was preventable, they are saying it should not have been allowed. Everyday English might just say you can stop it, or it need not happen.