Airships are a type of aircraft, a subclass of aerostat, that rely on light-than-air buoyancy for most of their lift characteristics, but unlike balloons, have powered-movement and are able to go from location to location without being borne on the wind. There are three types of airships, non-rigid airships or blimps, semi-rigid airships, and rigid-airships or dirigeables.
The genericized trademark zeppelin in the specific case refers to dirigeables, and in the non-specialist sense, refers to all airships. Blimps in the specific sens are only non-rigid airships, but in the general sense also applies to semi-rigid ones, and in the non-specialist refers to all airships. Dirigeables in the specific sense refers to only rigid airships, but in the general sense refers to also semi-rigid airships.
The buoyancy liftgas used in airships are typically hydrogen, helium, or hot air, or some combination of these. Theoretically, lowered density air or vacuum can also be used for lift, but the weight of the container for such a system is prohibitive. To counteract lift, these vehicles also contain ballast, which can be dumped to increase lift, and venting systems to decrease the liftgas to decrease lift. Ballast is usually sand or water, though some systems use air, or lowly compressed air, which is easily replaceable from outside air.