Om – the symbol of Brahman, tat – that singular Reality (Brahman), sat – the ever abiding presence (Brahman).
Thus ends the tenth chapter entitled Vibhūti Yoga, during the Srikrishna-Arjuna dialogue in Śrīmad Bhagavad Gita, constituting Yoga-śāstra, which falls within Brahmavidya as presented in the Vedic Upanishads.