Much has been said and written on the subject, and there are many opinions.
Here my definition of SteamPunk:
"Steampunk is a retrofuturist fantasy genre celebrating the Victorian Era and objects from that era (machines, fashion, etc).
The genre also permits the introduction of objects and persons from other times into the victorian era."
It may help, if you imagine the fictional works of Jules Verne and H. G. Wells, to be FACT.
The genre finds expression in all forms of art, including: literature, film, music, sculpture, painting, graphic art, fashion and jewelry design, among others.
My 2 cents.