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YALSA defines steampunk as “a literary mashup of science fiction, fantasy, and Victorian sensibilities.”
The line between steampunk and neo-Victorian is definitely blurry, but here’s a good example of the distinction in popular works:
In Scott Westerfeld’s Leviathan, the setting is an alternate WWI and the focus of that world is machinery– specifically, technology/science (often in the form of creatures) that did not exist at the time, but is used to build an alternate world.
In Libba Bray’s A Great and Terrible Beauty, the setting is a girl’s boarding school in the Victorian era, and the social and political world of Victorian England are well researched to inform the setting, dress, language and sensibilities of the characters. The supernatural plot elements do not alter the larger scope of the world enough to make it truly alternate in the classic, sci-fi definition, (which applies more aptly to steampunk).
Even though both kinds of work employ world-building, steampunk uses it to create an entirely alternate history that could not have existed, while the neo-Victorian uses world-building to create an alternate reality within a historical framework.
There are always exceptions to a rule like that, but it’s a decent place to start, especially since the two sub-genres tend to attract similar audiences.