Macromedia's (later Adobe's) animation software and browser plug-in, released in 1996. It defined interactive web art and design in the early 2000s; support ended in 2020.
Personally, Flash was such an important gateway for learning creative coding. There was this web zine called K10k and every week they would have a different artist take over.
Zach Lieberman, Zach Lieberman on the Resonance of Generative Art
I mean, we talk all the time about Flash ActionScript and how many amazing pieces used to be created through Flash. And now many of those pieces can never be run again because there's no support for it on any browser.
Conrad House, One Year of Le Random