I don't think Aikido is defined by techniques as much as it is defined by the philosophy of it's founder.
So what makes an aikido technique, aikido is the mindset/spirit in which the technique is performed.
Aikido is a methodology of training that has been imparted down to us from O'Sensei. To me Aikido doesn't have any exclusive techniques, only it's methodology and philsophy. Albeit, we typically center our studies around a core group of techniques. These techniques can be seen in tai chi, karate, aikijutsu, yoga, and even in the WWF!