Steampunk can be most closely described as a mixture between industrial steam-powered machinery and 19th century British Victorian style. It is a very striking and recognizable aesthetic often envisioned coming from an alternate timeline in which the modern era never made the switch from steam to internal combustion engines.
The term steampunk was first coined in the late 1980’s and pulls from works of fiction by H. G. Wells and Jules Verne. It gives a feeling of a tinkering alchemist in his workshop cooking up a potion. It has appeared in many books and tv shows.