I would say a real Amiga is anything that uses Amiga libraries to run Amiga software in hardware.
So an AmigaOne x5000 would count, because it's running Amiga (OS4) libraries to run Amiga software directly on the hardware.
Though by this definition, AROS would also be an Amiga, since the libraries are reverse engineered to run on different architectures.
If you wanted to be more stingy on the definition, make it require Kickstart in chip form (that would exclude the NG systems I think, and any FPGA one). Meh, an Amiga is whatever one wants to define it as.
