Ösel (yoga)

Luminosity or clear light (Tibetan od gsal, Sanskrit prabhāsvara), in Vajrayana, Tibetan Buddhism and Bon, refers to the nature of mind experienced in deep sleep and death.

Patrul Rinpoche defines ground luminosity as the:

...luminosity that occurs during deep sleep, union and death.

The practitioner trains to consciously enter the deep sleep state. If one has the ability to remain conscious during deep sleep, one will be able to recognize the luminosity of death and gain Buddhahood. This is called the meeting of mother and child luminosities, resulting in the state of thuktam at death.

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Wikipedia