Mahāvākyas

The Mahāvākyas (sing.: mahāvākyam, महावाक्यम्; plural: mahāvākyāni, महावाक्यानि) are "The Great Sayings" of the Upanishads, with mahā meaning great and vākya, a sentence. The Mahāvākyas are traditionally considered to be four in number, though actually five are prominent in the post-Vedic literature:

1. Tat Tvam Asi (तत् त्वम् असि) – literally translated as "That Thou Art" ("That is you" or "You are that"), appears in Chandogya Upanishad 6.8.7 of the Sama Veda, with tat in Ch.U. 6.8.7 referring to *sat, "the Existent," and contextually understood as "That's how [thus] you are," with tat in Ch.U. 6.12.3 referring to "the very nature of all existence as permeated by [the finest essence]."
2. Ahaṁ Brahmāsmi (अहं ब्रह्मास्मि) - "I am Brahman", or "I am absolute" (Brihadaranyaka Upanishad 1.4.10 of the Yajur Veda)
3. Prajñānaṁ Brahma (प्रज्ञानं ब्रह्म) - "Prajñāna is Brahman", or "Brahman is Prajñāna" (Aitareya Upanishad 3.3 of the Rig Veda)
4. Ayam Ātmā Brahma (अयम् आत्मा ब्रह्म) - "This Self (Atman) is Brahman" (Mandukya Upanishad 1.2 of the Atharva Veda)
5. Sarvaṃ Khalvidaṃ Brahma - "All this indeed is Brahman"

Mahāvākyas are instrumental in Advaita Vedanta, as they are regarded as valid scriptural statements that reveal the self (ātmán), which appears as a separate individual existence (jīva), is, in essence, non-different (not two-ness) from Brahman, which, according to Advaita, is nirguna. In contrast, these statements are less prominent in most other Hindu traditions, which emphasize a qualified or dualistic relationship between the self and Brahman, whom they regard as saguna, often identified with Vishnu, Shiva, Shakti, etc.