Steampunk started off as a literary sub-genre, spawned from Cyberpunk by three authors. I’ll go into more detail about them in my article on the History of Steampunk with a chronological timeline of how the culture came into being.
To give a brief outline here, though, the term “steam punk” was first written down by one of those three authors – KW Jeter – in a letter he wrote to Locus magazine in 1987. However, he had been writing books in the steampunk style from 1979.
Many people will argue that HG Wells and Jules Verne are the most famous steampunk authors. But it’s worth considering that Verne and Wells were in fact Victorian Science Fiction writers and contrary to what some people would have you believe, steampunk is not the same as Victorian Science Fiction. You can read the article about that by following the link. Steampunk is Victorian inspired Science Fiction. In a nutshell, it is science fiction using elements and themes that have a similarity to the Victorian era and uses futuristic or modern day vehicles/props that bear an uncanny resemblance to Victorian vehicles/props.
Examples: Modes of transport may be airship, but with a modern form of propulsion. Pistols could have a similar shape – or even be modified – from revolvers or duelling pistols, but they could be ray guns. Head wear could be a top hat with some technology hidden in the top such as a Frontal Lobe Accentuator.
Steampunk doesn’t have to have these elements, though. If you’re an author, you could have your novel set in the far future. It could be a dystopian battleground and your protagonist could be a time-traveller sent back to the Victorian era to alter the future. You could have a hero who is an immortal being from the dawn of time and has lived through the ages and has just reached the Victorian age. Either could use a lot of various elements that may not have any actual traditionally thought steampunk themes to them. Your novel could alternatively be slightly after the height of the Victorian era and a visit from an alien civilisation has propelled technology at an alarming rate. The design and aesthetics would therefore remain the same.
Whatever you decide, don’t feel constrained by what people say steampunk is. When I posted the article “Steamnpunk is NOT Victorian Science Fiction” author William J Jackson mentioned in the comments section that he’s had people telling him how a steampunk novel should be. He said: “Since I got on FB and got in touch with (steam)punks, it’s been… Interesting. Many telling me I’m not steampunk if I don’t cosplay, write in Victorian England,etc. If it’s all the same then it stagnates. We have to add our own touch, our own flair. Victorian England is cool, but steam-powered Greece or Mars or Cloud City is too.” That comment sums up many things that are wrong with steampunk and how we have to open our minds to the vast alternative universe that’s out there to be plundered.
I’ve repeated myself several times, in various articles as I think it’s an important point to make: Steampunk is completely personal and how you interpret it is entirely up to you.