Prabhāvam (Sanskrit: prabhāva, from pra- = forth/excellently + bhāva = being/arising) means glory, greatness, or potency — the inherent and emanating power that flows from an exalted source. In Śrī Vaiṣṇava usage it has two primary applications: the prabhāvam of Bhagavān's divine Name (nāma-prabhāvam), and the prabhāvam of the Āchārya (ācārya-prabhāvam).
The nāma-prabhāvam is the central teaching of many Śrī Vaiṣṇava rahasya granthams and is illustrated in the Nāmāzhvār-tradition: the divine Names — Nārāyaṇa, Rāma, Govinda — carry within them the full potency of Bhagavān's grace. Unlike a ritual mantra that requires the aspirant to be duly initiated and mentally prepared, the nāma-prabhāvam operates even for those who utter the Name inadvertently (aparādha-kṣamā). Devotional texts cite the liberation of Ajāmila (who called his son 'Nārāyaṇa' at death) as the supreme illustration: the mere sound of the Name is sufficient. This is because the Name is non-different from the Named (abheda between nāmin and nāma).
The ācārya-prabhāvam refers to the transforming effect that the presence, words, and grace of a genuine Āchārya produce in the disciple and the wider world. Pūrvācārya vaibhava granthams (glory-narratives), such as those composed about Rāmānuja or Maṇavāḷa Māmunigaḷ, document instances where contact with the Āchārya — a glance, a touch, a teaching — brought about radical spiritual transformation in kings, scholars, and ordinary people alike. This efficacy is not the Āchārya's personal property but is the prabhāvam of Bhagavān flowing through the Āchārya by virtue of his utter transparency to divine grace.