It’s basically science fiction that uses a Victorian aesthetic. As the title suggests, there’s always some kind of steam-powered technology involved. It’s often fairly advanced technology (for example: robots, airships, cyborgs, overly complex weaponry, etc.), but it nearly always uses steam power. Also all steampunk stories seem to involve at least one person wearing goggles. Or a top hat. Sometimes both. Oh, and gears. Random gears everywhere. And there’s usually some robots involved. Sometimes even they wear goggles, though I can’t fathom why.