Is Buddha immortal? (in Mahayana)

As I know Buddhism, it says 'Nothing is permament' I just came across this in Wikipedia : > ...a Buddha is immortal. Even though he descends in the samsara to > preach Dharma and save sentient beings from suffering, his original body remains in a transcendent realm. That body will not die upon the death of the physical body of Buddha, and hence a Buddha is beyond arising and passing away... It is given as a Mahayana Perspective of the Lotus Sutta. So, what does this really mean? Is it that there exists something even after Parinirvana? Or have I understood it wrong?