  A tensor invariant is defined on a quaternionic contact manifold in terms of
the curvature and torsion of the Biquard connection involving derivatives up to
third order of the contact form. This tensor, called quaternionic contact
conformal curvature, is similar to the Weyl conformal curvature in Riemannian
geometry and to the Chern-Moser tensor in CR geometry. It is shown that a
quaternionic contact manifold is locally quaternionic contact conformal to the
standard flat quaternionic contact structure on the quaternionic Heisenberg
group, or equivalently, to the standard 3-sasakian structure on the sphere iff
the quaternionic contact conformal curvature vanishes.
