I think it is a genre of science fiction based on the principle that modern day design is still based on that of the steam-driven Victorian era before the internal combustion engine. Steam power seems to be manifest in all the technology from scientific gadgets to dirigibles. It seems to be an antiquated off-shoot of cyberpunk.
Edit: To answer your question about dates for Steampunk, the Victorian era would certainly be foremost. The Victorian Era lasted from the pass of the Reform Act in 1832 to the end of Queen Victoria's rule in 1901. Although, any date seems to be acceptable in an alternate history setting where steam power rose to prominence in a history where the internal combustion engine didn't exist. Though even those tales seem to romanticize other elements of design and style from the Victorian era.