Steampunk: "science fiction dealing with 19th-century societies dominated by historical or imagined steam-powered technology" (Merriam-Webster)
Steampunk is a part of science fiction, taking place in alternative history or fantasy worlds that employ steam technology to power the world. It incorporates a distinctive Neo-Victorian aesthetic of clocks and gears in both the world, furniture, and cosplay.
The steampunk setting is popular in western media and in Japanese media. In western media, we could see the steampunk aesthetic pop up in movies such as The Wild Wild West, Mortal Engines, and The League of Extraordinary Gentleman, and TV shows such as Doctor Who and Legend of Korra. Japanese media embraced steampunk culture and aesthetics, creating many well-known media such as Nausicaa, Castle in the Sky, Nadia, Steamboy, Howl's Moving Castle, Last Exile, Sakura Wars, Final Fantasy VI, Final Fantasy IX, Kabaneri of the Iron Fortress, and Fullmetal Alchemist.
Steampunk fashion uses Victorian-style clothing such as suits, top hats, long dresses. It includes accessories such copper jewelry, pocket watches, and goggles to enhance the period look. Steampunk outfits tend to use muted hues such as black or brown to reflect Victorian Age colors. The outfits and accessories also include cogs, clocks, and sea creatures as part of the motif and design. Steampunk shows itself through cosplay in several ways, the first is a steampunk version of a non-steampunk character. The second is a cosplay whose origins stem from a steampunk work or land.
As you see, there is a distinctive difference between steampunk fashion and steampunk cosplay. The former (pictured right) is more of an aesthetic and a means of expression. It is more of an original work that reflects the tastes and creativity of the person wearing steampunk fashion. The latter is more of connecting steampunk to already established characters and worlds. It is about a steampunk equivalent to established characters, or performative work on characters from steampunk works.