Ruffle, that @joshtynjala nodded to, gets included as a script on your website, or can run as a standalone application on your desktop.
It acts as the Flash player, but is different because it no longer relies on the user's browser to supply the Flash player, your website instead provides the Flash player by means of Ruffle.
I will admit, I was sceptical when I first heard about this, but now I understand it a bit more, I think it's a brilliant idea. It's the sort of innovation Adobe regrettably hadn't thought of, before submitting Flash plugin to the grave Apple dug for it.
It's still a work in progress, particularly AS3 support.