Ninjedi is either 1.) A Ninja who is part Jedi, or 2.) A Jedi who is part Ninja. The Ninjedi is very rare, and one has never been seen. There have been so called sightings, but immediately after telling of these sightings, the person disappears.

A ninja who is strong with the force (actualy just STRONGER)
OR a Jedi who is more agile and stealthy than a master (sometimes there Sith more than Jedi though but still retain there name of Ninjedi due to there way of attack....stealthy)