Mahaavaakyam (महावाक्यम् plural = महावाक्यानि) = great utterances. These are the great singular statements or sentences from the Upanishads that encompass the entire paradigm of belief, or God. These mahaavaakyas are said to be the heart of Vedanta, the typical representation of Advaita philosophy (non-dual = there is only One).
The Vedas and Upanishads are written in cryptic language that requires deep study and contemplation to unlock both the meaning and impact of the verses. These mahaavaakyas exemplify the depth of knowledge that can be contained in cryptic statements of the scriptures.
Four principal mahaavaakyas, one from each of the Vedas, have been used since time immemorial to express the one (singular) yet universal message contained within the concise statements. The shukarahasya Upanishad v22 mentions these four mahaavakyas for meditation upon. These four principal mahaavaakyas are Pragyaanam brahma, Ayam aatmaa brahma, Tat tvam asi, Aham brahmaasmi. However, the rishis have recognised a number of mahaavaakyas for our contemplation and meditation.
These great utterances are meditative phrases that can lead to self-realization. Japa, with these mantras, is recommended to reach the stage where one can wholly grasp the meaning and impact of these phrases.
Brahman is Intellect (pure consciousness); Intellect is Brahman
I am Brahman
That thou art
अयम् आत्मा ब्रह्म4
Ayam aatmaa brahma
This soul is Brahman
(So-aham) I am that
सर्वं खल्विदं ब्रह्मम्6
Sarvam khalvidam brahma
All this is Brahman
एतद्वै तत् 7
(all of) this truly is that
He is one only, without a second.