That which has been called the Unmanifest and Imperishable, has been described as the goal Supreme. That is my highest state, having attained which there is no return. (Bhagavad Gita 8.21)
That is the Purusha, beyond manifest and unmanifest, by whom the whole universe is filled. That is the highest Goal. Evolution ends in That. He who has reached That is no longer subject to rebirth. He has crossed the sea of samsara. He has reached immortality, nirvana. This is not a state of existence; it is Existence itself, Consciousness itself, the highest Deity.
Source - Reflections on the Bhagavad Gita by Swami Atulananda – Prabuddha Bharata December 2003 issue.