I tell them it's fiction based on the period between both world wars. Dieselpunk can also be sci-fi or fantasy with that time period as the backdrop. Stories featuring airships, ray guns, daring adventurers exploring ancient ruins, or maybe just a simple detective working a hard case.
You know? In real life nobody has ever asked me what dieselpunk is. Here in Italy readers just don't know the word éè
But when someone asks me online, I answer that dieselpunk is a speculative genre inspired or set in the a time periode between the very late 1910s and the very early 1950s, that is punked with some kind of subversion. That may be fantasy of any kind twisting the reality we know, but it may also refer to a subtler kind of subversion.
My super quick thumbnail is, "what steampunk is to the Victorian Era, dieselpunk is to the early 20th Century."
I usual say that it's speculative fiction inspired by the 20's, 30's and 40's, with strong vibes from film noir, art deco, pulp magazines, and Lovecraft. Telling them about Sky Captain is an easy example.
I tell them that Dieselpunk is a lifestyle and subculture that combines elements of the culture of the 1920s to 1940s, in particular design, music and fashion, with today's technology and integrates these elements into modern life. The 'punk' in Dieselpunk means that it is not about historical correctness or nostalgia but just to mix old and modern styles in a way to have fun and give you a good feeling. People easily get what I mean by the example of my person. While, on the street, I always wear a dress shirt, a jacket, and a fedora, I'm also a modern man using the internet and a smartphone.