Who defines then what this really aikido is?
I tried to say that I understand it just the other way round: The name aikido is used for many ways of practice. But this ways of practice have become more and more different.
Who is "they"?
For the first question the answer is obviously:the art's basic principles, techniques, and the art's training menu and method as they were taught by o'sensei. Being aware of your centre, extending and using the ki, leading the oponent without resisting, moving in certain orbits in order to achieve that e.t.c...I have seen a lot of people claiming that they're doing aikido and the only thing in common was the...outfit and sometimes not even that! Blocking, grappling, pulling, pushing, not doing technique just hitting, claiming there is no ki, selling pills with ki( no kidding!)...you name it. And of course none of the above is aikido, yet the instructors were claiming that they were teaching different styles of aikido. So is it their word against mine? Probably not. One has to take a look at real aikido teachers (o'sensei, Steven Seagal sensei, the current doshu Moriteru Ueshiba just to name a few, there are others as well of course) to see the difference.
As for the second question, "who is they?"...The teachers who are claiming that they are doing another style of aikido than the original of course. They chopped aikido to pieces and then chose to study only what they like. They are not practicing the way o'sensei taught and then they are wondering why it is so difficult to achieve his level in aikido. Where's the logic in that? It really beats the hell out of me!