To denote that Brahman and Atman are One, the Chāndogya Upanishad uses the term Tat tvam asi, meaning ‘Thou art That’. In modern times, Nisargadatta Maharaj reiterated this fundamental truth of human existence in I Am That, which has been described by Vijai Shankar, an Advaita sage and former medical practitioner, as the only spiritual book you need to read.
Sanskrit tat ‘that’, from PIE base *tod-, extended form of *to-.