After a certain level of sophistication in computer hardware and moreso in software was reached, it's kind of pointless to define exactly what is or isn't an Amiga.
If 'Amiga' were to be defined exactly, that description could only point to one particular revision of one model of Amiga. So, if you wanted, you could use a set of exact definitions that cover every revision of every model or you could use a less exact, more flexible definition that allows what we know as 'Amiga' to grow and evolve.
Growing is painful but, in retrospect, a good thing. Evolution has some advantages too. :-)