A purifier is something used to take dirty or harmful substances out of air, water, or another material so it is cleaner. The ones students meet most are machines: an air purifier on a desk, a water purifier under the sink. The word is simply “the thing that purifies.” It is a practical, shop-window word as much as a science word. A filter is often the part that does the work; people still call the whole machine a purifier. You can, more rarely, call a person a purifier if their job is to clean something, but the machine is the usual picture. It does not by itself tell you how well the thing works — only that cleaning is the claim.