Does anyone know of a technique which exists only in Aikido?
To the best of my knowledge, I have seen variation of most techniques in more then one other style, and I got to see those without exploring for them ...
I doubt that there is technique that is unique to Aikido. There is a set of techniques (with some fuzziness on the boundaries) that
are Aikido techniques. I guess that this set is pretty much determined by the grading requirements.