“Steampunk is a sub-genre of science fiction that typically features steam-powered machinery, especially in a setting inspired by industrialized Western civilization during the 19th century … Steampunk perhaps most recognizably features anachronistic technologies or retro-futuristic inventions as people in the 19th century might have envisioned them, and is likewise rooted in the era’s perspective on fashion, culture, architectural style, and art. “
Wikipedia says that the film of Mervyn Peake’s “Titus Groan” anticipated many of the tropes of Steampunk. Other films that have had a strong influence on Steampunk include Fritz Lang’s Metropolis, and Terry Gilliam’s Brazil.
So, the key elements seem to be a Victorian feel, and an old fashioned idea of what the future will be. Jewelled or otherwise “scientifically” modified insects seem to play a reasonably large part, too. And dark lensed, round eyed goggles and glasses.