Dieselpunk

Dieselpunk is like a Steampunk, but instead of Industrial revolution we got both World Wars and period between them.

Aesthetics

If steampunk is all about brass, steam, blimps and clockwork robots, then dieselpunk is steel, internal combustion, tanks and dreadnoughts (of the naval kind, and the walking boxes kind).

Whereas Steampunk borrows the fashions and aesthetics of the Victorians, Dieselpunk's primary aesthetic is Art Deco. Anything not related to war machines will be very ritzy and flashy, and jazz to be the dominant form of music.

The overall technology level hovers around 1940: we got machine guns, cars, piston planes (and jets, but they're bleeding-edge tech that isn't found outside the military or corporations), radios and black-and-white TV sets. Electronics are something of a gray area: they exists, but not very advanced; you can expect radio, radars, alarms, simple encrypting/decrypting machines and so on, but no portable phones, detectors, transistors and computers (barring occasional ENIAC-styled vacuum tube monstrosities that take up at least an entire room). In a nutshell, if it looks like your grand-granddad would like it, that's steampunk, if it looks like it belongs to your grandpa, that's dieselpunk, and if it looks like your dad used it, that's early atompunk.

Also worth mentioning that there is significant difference in mood: steampunk stories often lean on romantical side of things, adventuring with science of the future and all, while dieselpunk goes full grimdark with industrialised, automated warfare, totalitarism and nihilism; steampunk is "The Time Machine" and "20,000 Leagues Under The Sea", dieselpunk is "All Quiet On The Western Front" and "Catch-22".

Examples

• Imperium (and, by extension, Chaos and Orks, since they like to loot their tech) is embodiment of this. Rule of thumb: if it doesn't look like it came out of Renaissance Fair or cheap 80'ies sci-fi show (or both), it's probably it. Imperial Guard is literally runs on it, special mention goes to Kriegers, Armageddoners, Mordians, Valhallans and entirety of Commissariat.
• Generally, every Russian or Russian-equivalent faction in sufficiently advanced setting is this; see Khador, Red Bloc; good vidya examples are Soviets from Red Alert and China from C&C:Generals.
  • Nazis and similiary themed factions tend to share this property whenever they're not too busy playing with occultism, eugenics and Tesla weapons. Kinda understandable: would you look at these penis compensators.
• In /v/, examples are Fallout (albeit if you're inclined to split hairs, that's atompunk (50'es to 70'es)), first two Bioshocks (Infinite is more steampunky), Dishonored and Wolfenstein (from "Return to" and onwards).
• For weeaboos goes non-magical bits of Fullmetal Alchemist, half of what Miyazaki did, Avatar's Fire Nation and whole Legend of Korra.