Gaslamp fantasy describes a parallel universe of 19th-century-level technology corresponding to our world’s Victorian age, but containing scientific and mechanical achievements in advance of Victoria's time—a modern version of the stories of H.G. Wells and Jules Verne, H. Rider Haggard, and other 19th-century authors of romantic adventure and super-science.
The term was coined by Kaja Foglio as a description of Girl Genius. The Foglios’ gaslamp fantasy universe is peopled with mad-scientist Sparks, their mechanical Clanks and biological Constructs, battling for world domination over common folks and to “Show them all!”.
I had been sorting through a lot of old sketches and noticed that Phil seemed to like drawing airships and weird creatures with walking sticks and old-fashioned clothes, but that stuff hardly ever made it into his final work. I've got a bit of a mania for that sort of thing myself, so it seemed a good idea to make a story that we could hang that look on. I called it Gaslamp Fantasy because, around the time we were bringing Girl Genius out, there was a comic called Steampunk on the shelves and I didn't want any confusion. Plus, I've never liked the term Steampunk much for our work, it's derived from Cyberpunk (a term which I think actually fits its genre well) but we have no punk, and we have more than just steam, and using a different name seemed appropriate. I mis-remembered a term that I had come across in the forward to an H. Rider Haggard book, where the author was talking about Jules Verne, H.G. Wells, Rider Haggard and that sort of pre-pulp adventure material, and came up with "Gaslamp Fantasy." I felt a bit foolish when I discovered that I had made up my own term, but it works and I like it.