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The Mahāvākyas (sing.: mahāvākyam, महावाक्यम्; plural: mahāvākyāni, महावाक्यानि) are “The Great Sayings” of the Upanishads, as characterized by the Advaita school of Vedanta with mahā meaning great and vākya, a sentence. Most commonly, Mahāvākyas are considered four in number,

Like other Mahāvākyas, it also explains the unity of Brahman and Atman, which is the basic principle of Advaita Vedanta.

Tat Tvam Asi (तत् त्वम् असि) – traditionally interpreted as “That Thou Art” (that you are), (Chandogya Upanishad 6.8.7 of the Sama Veda, with tat in Ch.U.6.8.7 referring to sat, “the Existent”); correctly translated 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]”
Ahaṁ Brahmāsmi (अहं ब्रह्मास्मि) – “I am Brahman”, or “I am Divine” (Brihadaranyaka Upanishad 1.4.10 of the Yajur Veda)
Prajñānaṁ Brahma (प्रज्ञानं ब्रह्म) – “Prajñāna is Brahman”, or “Brahman is Prajñāna” (Aitareya Upanishad 3.3 of the Rig Veda)
Ayam Ātmā Brahma (अयम् आत्मा ब्रह्म) – “This Self (Atman) is Brahman” (Mandukya Upanishad 1.2 of the Atharva Veda)
Those statements are interpreted as supporting the insight that the individual self (jīvá) which appears as a separate existence, is in essence (ātmán) part and manifestation of the whole (Brahman).