Nirvana literally means “to blow out.” In the Buddhist tradition, it refers to the extinguishing of the “three fires” of craving, aversion, and delusion. It signifies that the chains that bound you to conditioned existence have been broken; when you pass from this life, you enter into the deathless.
Henosis is the classical Greek word used in Neoplatonism for mystical “oneness,” “union,” or “unity” with the Ultimate Reality, called the One. It is an experience that can occur in this life, but is only fully realized when you pass from this life and return to the One.
What few realize is that Gotama and Plotinus are describing the same experience—the same realization of liberation from the conditioned realm. Both articulate two realities: the conditioned realm of becoming and the unconditioned reality of Being. The point is to cross from this shore to the far shore.
The reason this is not that clear is that they are approaching reality from different perspectives. Their civilization and cultural backgrounds are different; they are looking at the same ultimate reality from different angles.