A type of aquarium filter in which water either trickles or is sprayed on some sort of biological filter media, which has a large surface area so it can support large colonies of the beneficial bacteria that filter the water by converting ammonia to nitrites and nitites to nitrates, which are not nearly as toxic as the first two. The trickling or spraying of water oxygenates it, which makes the bacteria happy. Aka a trickle filter, wet-drys are usually made of acrylic and require a good-sized pump to run them, and are thus hideously expensive. You can make your own for far less that it would cost to buy one. They pull water out of the aquarium via a siphon overflow and usually have a sump at the bottom for stuff like UV sterilizers, heaters, and protein skimmers.