UrbanDictionary.com provides as good a range of definitions as any, as many as 11 by a current count.
Here's the top voted response:
Steampunk is a subgenre of speculative fiction, usually set in an
anachronistic Victorian or quasi-Victorian alternate history setting.
It could be described by the slogan "What the past would look like if
the future had happened sooner." It includes fiction with science
fiction, fantasy or horror themes. Medieval Steampunk: Speculative
fiction set during the Middle Ages. Victorian Steampunk: A modern
Science Fiction work (post-1930s) that is set in the early parts of
the industrial revolution. Western Steampunk: Science fiction set in
the American Old West. Industrial/Modern Steampunk: Science fiction
taking place in the late industrial age, early modern age; i.e. World
War 1, World War 2
Examples of steampunk:
Wild Wild West, Final Fantasy 7, Van Helsing, The Time Machine, Hellboy, Sky Captain and the World of Tomorrow
The second most popular definition hits on several elements that are essential to a full understanding:
(contains) prominent elements of either science fiction or fantasy,
such as fictional technological inventions like those found in the
works of H. G. Wells and Jules Verne, or real technological
developments like the computer occurring at an earlier date
Interestingly, in 2013 an IBM group's Social Sentiment Index prediction of cultural trends claimed that steampunk was soon to emerge as a mainstream pop culture force. E.g., see this blog post ... https://cultureby.com/2014/08/steampunk-cometh.html IBM was wrong as this Google Trends graphic based on steampunk as a keyword demonstrates:
The metric spikes kind of cyclically around Halloween each year after 2013 but clearly shows a relentlessly downward trend.
That said there are more interesting examples in cultural production where steampunk aesthetics have played a role. These include:
William Gibson's novels, Beastie Boys' Pauls Boutique, Lang's Metropolis, Riddick, Mad Max: Fury Road, Cirque du Soleil Kurious, John Wick 2, Eiffel Tower, David Lynch Dune, Barcelona’s Umbraculo, a vast greenhouse in the Parc de la Ciutadella, steampunk Shakespeare: TNT’s Will, Game of Thrones, The Mummy (2017), The Dark Tower (2017)
Finally, there's this Youtube vi, What’s so punk about steampunk?