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