Just as the title says:
What is the easiest definition of a Hyperkahler Manifold?
Could you give some examples of Hyperkahler manifolds, and manifolds which fail to be hyperkahler?
Why are such manifolds considered to be interesting in physics, and how do they arise in the study of supersymmetric gauge theories?
Apart from SUSY, are there any other branches of physics in which Hyperkahler manifolds appear?