...but, apparently, you're not interested in how we define "art." Why?
Nishio Sensei didn't create his own ryu of Aikido because he was dissatisfied with Aikido as an art form but as budo. Aikido's critics do not question it as an art form, they question it's combat/martial effectiveness. This is why I wanted to know whether there is a universal understanding/definition of what martial is, because it is important to the debate within the martial arts community. I've read people criticise Aikido for the techniques requiring too much compliancy to work, I've read criticisms for its apparent lack of atemi, I've read criticisms for its lack of competition and therefore the techniques being "untested". I have never read of anyone who criticises Aikido as an art form, for it not being sophisticated enough or not aesthetic or not philosophically satisfying etc.
For me Aikido is the epitomy of a martial art. It's techniques come from well established jutsu arts and their adaptation has resulted in a very pure, even beautiful use of blending and harmonization with your opponent's energy that is both martial and humanitarian. To watch Aikido being demonstrated is a wonderful experience, to see the waza being executed masterfully and their resulting effect in protecting uke and nage is truly art in motion.