I always thought the Wikipedia definition was accurate:
Steampunk refers to a subgenre of science fiction and sometimes fantasy that incorporates technology and aesthetic designs inspired by 19th-century industrial steam-powered machinery.
I wasn't too confused or worried when it also started being used for worldbuilding that had a lot of similarities to the 19th century Victorian era fashions, clockwork devices, attitudes, steam machines, etc. Or across the pond here the American Wild West. What you get with Mark Twain, H.G. Wells and Jules Verne. Somewhat coined in contrast to "cyberpunk.". The look and feel of the new Sherlock movies and series. A very good explanation, in terms of books, from one of today's bestselling steampunk author at http://www.ministryofpeculiaroccurrences.com/what-is-steampunk/ . (Yes, I know it is also used for cosplay, fashion and other areas but in terms of book genre it's getting over-used and losing meaning).
A little less on board now that it seems anything not an advanced technological society or a contemporary setting with today's technology in place is somehow steampunk.
I swear booksellers are just putting everything not today's or futuristic advanced technology as steampunk, particularly if not fitting well in other categories and particularly because it's become a top selling subgenre and site search term. I don't usually care for steampunk, so it turns me away from a lot of books I'd otherwise jump on (or makes me ask around and sample first). I realize other readers cannot get enough of it and it sells well — no doubt why booksellers like the category — but almost never my cup of tea.
Seriously, I'm reading an alternate history of the 1763 Indian Wars (colonial America before Independence) and it's being called steampunk. Another BoTM I've been leery of starting because was labeled steampunk ("Three Parts Dead") but friends/followed and BoTM participants all say it is not steampunk and has nothing to do with 19th century Victorian anything.
It's Amazon, kobo, bn, diesel, publishers and others all guilty of steampunking books that aren't. I like many aspects of the redesign of Barnes and Noble website, including seeing genre. But oh boy are they in pparticular guilty of steampunking crap--and listing lots of books as erotica that aren't (even for some weird reason some children's and middle grade books).