If you see someone asked that question on TV, or read quick definitions online, you usually get something to the tune of "fiction in a 19th Century setting" and then something like, "with all the imaginary technology that never-was" or "with retro futuristic technology" or "if the Communications Revolution happened 100 years earlier" etc. Then they go on to describe devices like Nemo's Nautilus, or armored airships and zeppelins filling the skies, steam-powered robots, gear-powered prosthetic limbs, goggles with specially advanced lenses, or artificially intelligent Babbage engines (analog computers) as big as a room with contacts made from brass.
I say they are half right. They have the technology part down, but saying Steampunk is only about the technology is like saying cyberpunk is only about the technology.
Here's how I define it in my game.
Steam = “Steam” in this case sets up the technology of the world, and to some respect, the historical context of that technology. The two simple rules of thumb are
1) If it is mechanical in any way, it can run off steam or gears, and
2) Any scientific theory can be considered reality if it sounds convincing enough in a drawing room over a snifter of brandy.
I then go into all the wild scientific theories that go with that. For example, The earth is hollow, iron clad automatons and robots are possible, cloning and other weird bio-science is possible (don't forget Frankenstein), it is easy to modify people using steam or gear operated mechanical devices, there is air on all planets and moons, the space between all things is the Aether, time travel is possible, Babbage Engines can do anything, and anything (no matter how big or heavy) can be made to fly or swim.
I further expand that with "Steam" comes the sensibilities and aesthetic of the 19th Century, usually, but not always like Victorian London. While a Steampunk story could take place in 1800s England, it could also take place in the United States (especially the Wild Wild West), or other countries, or in alternate timelines, or in completely different worlds altogether.
Regardless of where or when it takes place, there are still some common themes to keep within the Steampunk aesthetic.
1) There are significant and radical differences between the classes, the genders, and in some cases, the races (whether or not we are talking about ‘race’ in a colloquial sense of different ethnicities, or actual different species of creatures like tiger-people, or aliens, or fairies). Most protagonists will tend to be Egalitarians, above such trivial distinctions.
2) New advances in food production and medicine mean massive overcrowded cities. These cities will almost always have at least one set of slums.
3) Expect that prostitutes, scientists, alcoholics and orphans will play a significant part in any setting.
4) Likewise, expect factories, mines, mills, secret societies and guilds to play a significant part in any setting.
5) There may or may not be factions of people (sometimes referred to as Luddites) who violently oppose technological advances, fearing they will lose their jobs to it.
6) Any technology you would see made of out plastic today, would in a Steampunk setting be crafted from hand-carved wood (Mahogany and Rosewood are favorite choices), brass, copper or iron.
7) The dress, architecture, and manner of speech of a Steampunk world will be guided at least in some respect by those from historical Victorian times.
Punk = In most cases, “punk” refers to people striving for individual freedoms in a world where such things are generally oppressed or otherwise put down. The setting will define how oppressive the government and the society are. The characters will have their own ways for how they personally “rebel” against whatever travesty it is they choose to rebel against. Long story short, the more emphasis on the “Punk” part of the world, the less pleasant a place it is to be. Gritty, dark, and entirely free of happy endings might be a good way to think of it.
When you see people in Steampunk attire, they aren't always just wearing Victorian suits or dresses and goggles (although some do). They have tattoos, piercings, and dyed hair as part of the outfit, none of which would have probably happened in 19th Century London. They dress with leather bracers, they carry large impossible guns, they have insignia with skulls and wings - in short, air pirates are very popular. These all hint at the "rebellion" or "punk" aspect of the genre. It is about fighting for or against something bigger than oneself.
In fiction, you'll see things like,
1) Women might wear men’s clothing, or they might wear comfortable or practical clothing, and actually participate in science and adventuring.
2) People carrying weapons all the time
3) People modifying themselves for appearance or function with mechanical alternations such as artificial limbs or eyes.
4) Displaying gadgetry and inventions on their person, such as powered gauntlets, mechanical wings, or technologically enhanced goggles.
Some common things for Steampunk characters to rebel against
3) Non-Representative governments
4) All politics or government, or simply the taxes which pay them.
5) Abuse of human rights
6) Abuse of rights of privacy
8) The unequal distribution of wealth
9) The celebration of debauchery and self-indulgence, particularly by the wealthy.
10) The “ideals” or “morals” of society
11) Exploitation of any group
15) Secret Societies
16) Wars or misuse of military
There are three core Steampunk world views:
1) STEAMpunk (heavy on technology, light on punk)
2) steamPUNK (focuses on the oppression and darkness of the world)
3) Fantasy Steampunk (incorporating fantastic races and creatures such as fairies and goblins, along with magic mixing with technology).
When choosing a world view, feel free to mix and match between the options. For example, to an aristocrat who has never left the "good” side of town, the entire world might seem more in line with STEAMpunk until that day a mysterious messenger leads them into the city slums, or they lose their entire fortune in a series of bad investments and learn the hard way the brutal living conditions of the lower classes. Likewise, a Fantasy Steampunk campaign could have roots in a utopian society just as easily as a dystopian one.
A STEAMpunk world is one where not only has technology progressed faster than the historic pace, but it has done so to the benefit of society in general. While there might still be a significant economic difference between the classes, food is plentiful, and cities relatively clean. A lot of the Japanese animated Steampunk movies fall in this category.
steamPUNK More of the written fiction falls in this 'darker' category. A steamPUNK world is one where the technological advances primarily benefit the upper classes, the government, or the military, many times at the expense of the lower classes. The economic difference between the classes is harsh and cruel. Take anything positive from the STEAMpunk list above and add the phrase “only for the upper class,” to the end.
Fantasy Steampunk While putting “Fantasy” in front of Steampunk may seem redundant, in this case it is used to incorporate a specific element not common in most Steampunk literature. Given the popularity of the belief in fairies during the 19th Century, it isn’t a giant leap to add such creatures of the imagination into a Steampunk world. Also, the 19th Century was a time of incredible scientific advances happening quicker than most people could grasp them. There was no reason not to believe that if an armored zeppelin could fly, why couldn't a little girl on a broom? Since advanced technology is essentially "magic" to people who don't understand even the fundamentals behind it, the idea of actual arcane magic existing beside new technological marvels seems to make sense.
Besides, orks with cannons for arms? Female elves in Steampunk garb with blaster pistols and pointed ears? Fairies (or their skeletons) on display in jars? Using a gear-powered box to predict the future or speak with the dead? 19th Century zombies and vampires? That's just cool stuff...
Here is a (very small) list of some sample Steampunk books...
• Steampunk (Collection of short stories edited by Ann and Jeff Vandermeer)
• Steampunk’d (Collection of short stories edited by Jean Rabe and Martin H. Greenberg)
• Extraordinary Engines (Collection of short stories edited by Nick Gevers)
• The Steampunk Bible by Jeff Vandermeer with S. J. Chambers
• Boneshaker by Cherie Priest (Steampunk AND zombies…)
• The Difference Engine by William Gibson and Bruce Sterling
• Leviathan by Scott Westerfeld
• The Girl Genius series by Phil and Kaja Foglio
And Movies/TV Shows with at least a Steampunk theme
• 20,000 Leagues Under the Sea
• Howl’s Moving Castle
• The Castle in the Sky: Laputa
• The League of Extraordinary Gentlemen
• Sherlock Holmes
• Sky Captain and the World of Tomorrow
• The Wild Wild West
• The Island of Dr. Moreau
• The Time Machine
• The City of Lost Children
• Warehouse 13