Brahman in the Upaniṣads
Brahman (Sanskrit: ब्रह्मन् — from bṛh, 'to grow, to expand') is the central concept of the Upaniṣads: the one ultimate reality from which all else derives its existence. Multiple Upaniṣadic passages describe Brahman as sat-cit-ānanda (existence-consciousness-bliss), as sarvam khalv idam Brahma ('all this is Brahman'), as the inner ruler of all.
Brahman = Śrīman Nārāyaṇa
The most fundamental claim of Śrī Vaiṣṇavism is the identification: Brahman = Śrīman Nārāyaṇa (not Śiva, not Brahmā, not an impersonal principle). Rāmānuja's Śrī Bhāṣyam demonstrates through systematic textual analysis that every Upaniṣadic characterization of Brahman — omniscience, omnipotence, being the cause of all, being the Lord of all — applies uniquely to Śrīman Nārāyaṇa with Śrī as His inseparable consort.
The Qualified Brahman
Viśiṣṭādvaita's Brahman is the 'qualified Brahman' (sa-viśeṣa Brahman): not a featureless Absolute but a supremely personal God with infinite auspicious qualities (kalyāṇa-guṇas) — omniscience, omnipotence, compassion, beauty, accessibility, lordship, and infinite more. Advaita's 'nirvikalpa Brahman' (without qualities) is rejected as a misreading of the Upaniṣads.
Brahman's Body
Rāmānuja's revolutionary formulation: Brahman is not alone — He eternally has chit (souls) and achit (matter) as His body (śarīra). The formula 'Brahman' thus always means: Śrīman Nārāyaṇa together with His infinite worlds and souls as His body. This is the Viśiṣṭādvaita meaning of 'one without a second.'