I propose a rule of thumb to be used as a pragmatic definition of a superintelligent system. It is meant to be practical to a degree where everyone can have a shared understanding if a system has reached superintelligence or not.
A system is superintelligent if in a remote hiring process it can get any non-physical job, earn a salary and perform its duties to the satisfaction of the employer. This should be true for all positions and for all levels of seniority, from an intern to senior staff and PHDs. The employer must believe they are employing a human (it's ok if doubts arise due to system vastly outperforming its peers).