Etymology: steam + punk, by correlation to ‘cyberpunk’
(science fiction genre, uncountable) A subgenre of speculative science fiction set in an anachronistic 19th century society.*
(countable, cosplay) A person cosplaying as a steampunk character.
* “It is also the vision of steampunk, a subculture that is the aesthetic expression of a time-traveling fantasy world, one that embraces music, film, design and now fashion, all inspired by the extravagantly inventive age of dirigibles and steam locomotives, brass diving bells and jar-shaped protosubmarines.” – May 8th 2008, Ruth La Ferla; Steampunk Moves Between 2 Worlds, New York Times
What can you see from here?
To me the combination of Steampunk and art nouveau is a natural one. Steampunk is firmly embedded in a rather generalised form of Victoriana. For many Steampunks it seems to be enough to add bustles, bloomers, goggles and gears to modern art forms, but the Victorian era was the origin of a number of groundbreaking schools of art. Why not use those schools of art in the same way that Steampunk authors use the linguistic style of the period. Art nouveau is a purely aesthetic form of art that lends itself well to a style and genre like Steampunk, which so far seems to be more about visual impact than deeply symbolic statements. It also represented a move towards advertising as a form of artist expression and the concept that one could have as much selling impact with images as with words.