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