I'd argue that Arduino isn't even fundamentally a hardware thing. I can code an Arduino application (sketch) and upload it directly to an Atmel chip on a bread

I'd argue that Arduino isn't even fundamentally a hardware thing. I can code an Arduino application (sketch) and upload it directly to an Atmel chip on a breadboard, with no other components present at run-time. Atmel didn't make the chip as an Arduino (they made it to control toaster ovens or whatever), so in that scenario there's no Arduino except for their boot loader and runtime library.