You might be referring to the End Effector or the Root joint. Otherwise, if a joint has no connections, then your End Effector is also your Root joint. This would happen if you are trying to apply IK to something like an eye ball. In this case, an eyeball is a joint with no outboard connections and no inboard connections. An eye ball only rotates, and tries to follow a target, but it will most likely never reach it with just IK; however, it will look in the direction of the Target. Does this help?