To me, the question is simple. Steampunk is fiction inspired by the Victorian era (generally featuring steam technology) – and by “inspired by” I mean it is always deliberately anachronistic (especially in the area of fantastical tech). It is used to cover clockpunk (same thing, but pre-Victorian, when clock technology was the wonder of the age), and dieselpunk (post-Victorian, with lots of diesel tech especially zeppelins).
Here is an article on someone else’s take – featuring lots of great quotes. I’ve included this one because I agree with it:
Steampunk simply embodies a time and a place. The time… the late 19th century. The place… a steam powered world, where air travel by fantastical dirigibles is as common as traveling by train or boat (or submarine). A place where national interests are vastly different than our own version of history. A place where the elegant and refined are as likely to get pulled into a grand adventure, as the workers, ruffians, and lower classes. A place where the idea of space travel is not so far fetched. A place where lost civilizations are found and lost again. A place where anything is possible, and science can be twisted to meet ones own ends. That to me is the essence of Steampunk. It can have political overtones and commentary, or it can be straight escapist fiction. Either way, if it meets these criteria. It is Steampunk.
:-Joshua A. Pfeiffer a.k.a. Vernian Process
And here for your delectation is a slightly clockpunk (yes the clock is real, the entire table was made by hand, and it works) photo of my cat: