Nibbāna is directly visible in a provisional sense when one enters a jhāna or subsequent meditation attainments. It is directly visible in the definitive sense when one attains the cessation of perception and what is felt, and having seen with wisdom, completely exhausts mental defilements.
“‘Sandiṭṭhikaṁ nibbānaṁ, sandiṭṭhikaṁ nibbānan’ti, āvuso, vuccati. Kittāvatā nu kho, āvuso, sandiṭṭhikaṁ nibbānaṁ vuttaṁ bhagavatā”ti?
“‘|Directly visible::evident, perceivable [sandiṭṭhika]| |Nibbāna::complete cooling, letting go of everything, deathless, freedom from calamity, the non-disintegrating [nibbāna]|, directly visible Nibbāna,’ friend, it is said. But to what extent, friend, did the Blessed One say that the Nibbāna is directly visible?”
“Idhāvuso, bhikkhu vivicceva kāmehi …pe… paṭhamaṁ jhānaṁ upasampajja viharati. Ettāvatāpi kho, āvuso, sandiṭṭhikaṁ nibbānaṁ vuttaṁ bhagavatā pariyāyena …pe….
“Here, friend, a bhikkhu, quite secluded from sensual pleasures and |unwholesome::unhealthy, unskillful, unbeneficial, or karmically unprofitable [akusala]| mental states, enters and dwells in the first jhāna. To this extent, the Blessed One has spoken of the directly visible Nibbāna in a |provisional sense::qualified sense, in a certain respect [pariyāyena]|. ...
Puna caparaṁ, āvuso, bhikkhu sabbaso nevasaññānāsaññāyatanaṁ samatikkamma saññāvedayitanirodhaṁ upasampajja viharati, paññāya cassa disvā āsavā parikkhīṇā honti. Ettāvatāpi kho, āvuso, sandiṭṭhikaṁ nibbānaṁ vuttaṁ bhagavatā nippariyāyenā”ti.
Further, friend, with the complete surpassing of the |base of neither perception nor non-perception::field of awareness of subtle mental activity that do not arise to the level of forming a perception [nevasaññānāsaññāyatana]|, the bhikkhu enters upon and abides in the |cessation of perception and what is felt::ending of recognition and felt experience, cessation of conception and what is felt [saññāvedayitanirodha]|, and having seen with wisdom, his |mental defilements::mental outflows, discharges, taints [āsava]| are completely exhausted. To this extent, the Blessed One has spoken of the directly visible Nibbāna in the |definitive sense::literal sense, ultimate sense [nippariyāyena]|.”
“‘|Directly visible::evident, perceivable [sandiṭṭhika]| |Nibbāna::complete cooling, letting go of everything, deathless, freedom from calamity, the non-disintegrating [nibbāna]|, directly visible Nibbāna,’ friend, it is said. But to what extent, friend, did the Blessed One say that the Nibbāna is directly visible?”
“Here, friend, a bhikkhu, quite secluded from sensual pleasures and |unwholesome::unhealthy, unskillful, unbeneficial, or karmically unprofitable [akusala]| mental states, enters and dwells in the first jhāna. To this extent, the Blessed One has spoken of the directly visible Nibbāna in a |provisional sense::qualified sense, in a certain respect [pariyāyena]|. ...
Further, friend, with the complete surpassing of the |base of neither perception nor non-perception::field of awareness of subtle mental activity that do not arise to the level of forming a perception [nevasaññānāsaññāyatana]|, the bhikkhu enters upon and abides in the |cessation of perception and what is felt::ending of recognition and felt experience, cessation of conception and what is felt [saññāvedayitanirodha]|, and having seen with wisdom, his |mental defilements::mental outflows, discharges, taints [āsava]| are completely exhausted. To this extent, the Blessed One has spoken of the directly visible Nibbāna in the |definitive sense::literal sense, ultimate sense [nippariyāyena]|.”
“‘Sandiṭṭhikaṁ nibbānaṁ, sandiṭṭhikaṁ nibbānan’ti, āvuso, vuccati. Kittāvatā nu kho, āvuso, sandiṭṭhikaṁ nibbānaṁ vuttaṁ bhagavatā”ti?
“Idhāvuso, bhikkhu vivicceva kāmehi …pe… paṭhamaṁ jhānaṁ upasampajja viharati. Ettāvatāpi kho, āvuso, sandiṭṭhikaṁ nibbānaṁ vuttaṁ bhagavatā pariyāyena …pe….
Puna caparaṁ, āvuso, bhikkhu sabbaso nevasaññānāsaññāyatanaṁ samatikkamma saññāvedayitanirodhaṁ upasampajja viharati, paññāya cassa disvā āsavā parikkhīṇā honti. Ettāvatāpi kho, āvuso, sandiṭṭhikaṁ nibbānaṁ vuttaṁ bhagavatā nippariyāyenā”ti.