Nirvana or Nibbana is not a thought or concept of the mind. It is that which is experienced by the mind, when it is completely free of all fetters and defilements.
I quote from this answer below. Please see that answer for a detailed analysis including sutta quotes.
So, Nibbana is not a thought of the mind, not a concept of the mind,
not a state of the mind, not a state of consciousness and also not a
feeling. However, when the mind experiences this Nibbana, which is not
conditioned, not compounded, not suffering, not impermanent, not
arising, not ceasing and not changing, it experiences bliss. The mind
can therefore experience Nibbana, but it cannot feel it or think about
Sukha or happiness for an unenlightened person is experienced when
encountering pleasant feelings (from the six senses) or when
encountering the cessation of painful feelings (from the six senses).
But for an arahat, sukha or bliss (in this context) is experienced
when encountering neutral feelings, no feelings and Nibbana.
Also from that answer:
From AN 9.34 (translated by Bhikkhu Sujato):
Ven: Sariputta: “Reverends, extinguishment (Nibbana) is bliss!
Ven. Udayi: “But Reverend Sāriputta, what’s blissful about it, since
nothing is felt?”
Ven. Sariputta: “The fact that nothing is felt is precisely what’s
blissful about it.