What is an unpowered lighter-than-air craft called?

An unpowered lighter-than-air craft is called a balloon.

A balloon rises because its overall density is lower than that of the surrounding air. This lift comes from a gas such as helium or hydrogen, or from heated air. Unlike an airship, a balloon normally has no engine and cannot steer itself reliably through the air; it generally travels with the wind.

The word “balloon” covers several designs, including hot-air balloons, gas balloons, tethered balloons, and scientific research balloons. Hot-air balloons use a burner to heat the air inside the envelope, while gas balloons use a naturally buoyant gas. Hydrogen provides slightly more lift than helium but is highly flammable, so helium is preferred for many modern applications.

A common mix-up is calling every lighter-than-air aircraft a balloon. Powered, steerable lighter-than-air craft are airships or dirigibles. The broader technical category is “aerostat,” which includes both unpowered balloons and powered airships.