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Aerostats use buoyancy to float in the air in much the same way that ships float on the water. They are characterized by one or more large gasbags or canopies, filled with a relatively low density gas such as helium, hydrogen or hot air, which is lighter than the surrounding air. When the weight of this is added to the weight of the aircraft structure, it adds up to the same weight as the air that the craft displaces.

Small hot air balloons called sky lanterns date back to the 3rd century BC and were only the second type of aircraft to fly, the first being kites.

Originally a "balloon" was any aerostat, while the term "airship" was used for large powered aircraft — usually fixed-wing — though none had yet been built. The advent first of powered balloons, called dirigible balloons, and later of rigid hulls allowing a vast increase in size, began to change things. Huge powered aerostats, characterized by a rigid outer framework and separate aerodynamic skin surrounding the gas bags, came to dominate the skies, the Zeppelins being the largest and most famous. There were still no aeroplanes or non-rigid balloons large enough to be called airships, so "airship" came to be synonymous with these great monsters. Then several accidents, such as the Hindenburg disaster in 1937, led to the demise of these large rigid airships due to safety fears. Nowadays we say that a balloon is an unpowered aerostat, whilst an airship is a powered one.

A powered, steerable aerostat is called a dirigible. Sometimes this term is applied only to non-rigid balloons, and sometimes dirigible balloon is regarded as the definition of an airship (which may then be rigid or non-rigid). Non-rigid dirigibles are characterized by a moderately aerodynamic gasbag with stabilizing fins at the back. These soon became known as blimps. During the Second World War, this shape was widely adopted for tethered balloons; in windy weather this both reduces the strain...