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, or a post apocalyptic environment. Therefore, steampunk works are often set in an alternate history of the 19th century's British Victorian era or American "Wild West", in a post-apocalyptic future during which steam power has regained mainstream use, or in a fantasy world that similarly employs steam power.
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. Such technology may include fictional machines like those found in the works of H. G. Wells and Jules Verne, or the modern authors Philip Pullman, Scott Westerfeld, and China Mieville.
Other examples of steampunk contain alternate history-style presentations of such technology as lighter-than-air airships, analog computers, or such digital mechanical computers as Charles Babbage and Ada Lovelace's Analytical Engine.
Steampunk may also, though not necessarily, incorporate additional elements from the genres of fantasy, horror, historical fiction, alternate history, or other branches of speculative fiction, making it often a hybrid genre.
The term steampunk originated during the 1980s and early 1990s, though now retroactively refers to many works of fiction created even as far back as the 19th century itself.
[definition courtesy of Wikipedia]