Dirigible: Also known as airships, these are essentially streamlined balloons with a propulsion system. Dirigibles come three flavours: rigid, like the Hindenburg and its rivals, which had full airframes; semi-rigid, which have a keel or other partial structure; and non-rigid, which hold their shape by gas pressure alone, like the Goodyear blimps. Dirigible use peaked between the World Wars, but they may yet make a comeback on the strength of their ability to haul massive loads by air. While hot-air dirigibles are possible, helium or hydrogen-filled ones are more common.
Tech Level: The first powered flight was in 1852, and it was—drum roll, please—a steam powered dirigible. Go Steampunk! Airships are a defining trait of this genre.
Appeared In: My favourite Spec Fic anime: Nausicaä of the Valley of the Wind, and other wonderful Hayao Miyazaki projects such as Castle in the Sky. Kenneth Oppel‘s Airborn and its sequels Skybreaker and Starclimber are all on the shelf at my right hand as I write this.
For Your Plot: I’m biased here. Airships play a major role in my own work, where they provide trade between isolated communities on a lost colony planet. And impose trade sanctions, but that’s another story. You go write your own.