IMO, "Amiga" is the functionality and layout of the entire system. The OS has a great deal to do with what an Amiga is. Amiga is a fast, easy to use system. It's Workbench that doesn't get in the way and is simple. It's the heavy-feeling mouse that has a low pitch click rather than mice today with a high pitch click. It's the sound of the keys of the keyboard. It's the ability to play a full game from a floppy disk by turning on the computer and the game just works (try that on ANY other computer system--only Amiga can do that--even today). It's the way the OS and hardware work together seamlessly.
This is "Amiga." Without such characteristics, it isn't Amiga regardless of who makes what.
My $0.02.