The Mahavakyas are the Great Sentences of Advaita Vedanta and Jnana Yoga, and are contained in the Upanishads. Maha is Great, and Vakyas are sentences, or utterances for contemplation.
The Mahavakyas
तपः स्वाध्यायेश्वरप्रणिधानानि क्रियायोगः