The fourth (out of the five) Vimānas (habitats) heaven of the Anuttara Vimāna (the highest heaven of the Empyrean gods);
(the name of this Vimāna is Aparājita (lit. invincible)), as this Empyrean god who is its denizen is never overcome by the causes which bring about impediments in his aggrandizement; such Deva (god) is possessed of rarefied Karma; as he remains always contended, he is not vulnerable to hunger etc...
anuttarāḥ pañca devanāmān eva. vijitā abhyudayavighnahetavaḥ ebhiriti vijaya-vaijayanta-jayantāḥ. taireva vighnahetubhirnaparājitā aparājitāḥ.
(TaBhā 4.20)
pratanukarmapaṭalāvacchannatvāt... satatatṛptatvānna kṣudādibhiḥ parājīyanta ityaparājitāh.
(TaBhā 4.20 Vṛ)