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?
First, there are permanent phenomena and impermanent phenomena. All products are impermanent, whereas all negative phenomena are permanent. For instance, space, emptiness, mental images are permanent due to not being products, that is not arising due to causes and conditions.

Second, the meaning of permanent is that of not being subject to change. There are permanent phenomena that are not eternal but occasional: for instance, the emptiness of the cup no longer abides if you break the cup.

Then, there are various bodies. The Dharmakaya has two parts (bodies) :


The wisdom truth body, which is the omniscient mind of a buddha. As it is a mind, it is impermanent, it is compounded
The nature truth body, which is the emptiness of the omniscient mind of a buddha. As it is an emptiness - suchness without defilement - it is permanent and uncompounded



The Dharmakaya is endowed with four aspects:


Permanent, in the sense of not being subject to birth [not taking even a mental body, as opposed to an arya bodhisattva who takes a menal body due to uncontaminated karma and knowledge obscurations]
Constant, in the sense of not being subject to the inconceivable transformation of death [of a mental body] and therefore his enlightened deeds are un-interrupted
Peace, in the sense of being free from all harm
Unalterable, in the sense of being free from aging [contrary to even the arya bodhisattva on a pure ground]


Most Mahayana traditions consider Shakyamuni Buddha's body - the one ordinary being could see - as an emanation body (not a mental body), emanating from the pure dharmakaya.

For more information, See Maitreya / Asanga's Sublime Continuum and commentaries, such as Gyaltsabe Je's.