Brahman is a central Vedāntic term for ultimate reality. The Upaniṣads describe it in several contexts as the profound ground, source or essence of existence; later schools interpret those passages in importantly different ways. Chāndogya Upaniṣad 6.8.7 is a major textual reference for discussions of self and ultimate reality.
For Advaita, Brahman is ultimately nondual and the deepest ātman is not other than it. In Viśiṣṭādvaita, Brahman is the personal supreme, with real souls and world inseparably dependent on that supreme. In Madhva's Dvaita, Brahman is Viṣṇu, the independent reality, while selves and matter remain distinct and dependent. Brahman should not be confused with Brahmā, the deity named in many Hindu narratives, or with the social term Brahmin. Nor should any one school's interpretation be silently presented as universal.