Mūla-mantra — The Root Mantra
"Mūla-mantra" consists of Sanskrit mūla (root, foundation, origin) and mantra (sacred utterance, mystic formula). Just as the root of a tree sustains and gives rise to the entire plant, the Mūla-mantra is the foundational sacred utterance from which the practitioner's entire spiritual life takes nourishment and meaning. In Śrī Vaiṣṇava tradition, this title belongs to the Aṣṭākṣara: "Oṃ Namo Nārāyaṇāya" — the eight-syllabled mantra of the Lord.
The Aṣṭākṣara is the first of the three Rahasya-mantras (esoteric mantras) transmitted at Pañca-saṃskāra initiation: Aṣṭākṣara (Mūla-mantra), Dvayam (Tāraka-mantra), and Carama-śloka. As the Mūla-mantra, the Aṣṭākṣara encodes the entire philosophy of Viśiṣṭādvaita Vedānta in eight syllables. The praṇava (Oṃ) declares the soul's essential servitude; namaḥ negates any ownership of the self; Nārāyaṇāya identifies the Supreme Person as the sole refuge. According to the ācāryas, one who truly understands the Mūla-mantra has understood the whole of scripture.
Piḷḷai Lokācārya's Mumukṣuppāḍi opens with an analysis of the Aṣṭākṣara, examining the meaning of each component in order to establish the correct understanding of the soul's relationship (sambandha), the means of attaining liberation (upāya), and the goal to be attained (upeya). The daily repetition (japa) of the Aṣṭākṣara — received from the Āchārya at initiation — forms the backbone of a Śrī Vaiṣṇava's personal practice.