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What or where is Nirvana?

It is a dimension transcending time and space and thus is difficult to talk about or even think about. Words and thoughts being only suited to describe the time-space dimension. But because Nirvana is beyond time, there is no movement and so no aging or dying. Thus Nirvana is eternal because it is beyond space, there is no causation, no boundary, no concept of self and not-self and thus Nirvana is infinite. The Buddha also assures us that Nirvana is an experience of great happiness. He says:
Nirvana is the highest happiness.

Dhammapada 204