Total Sovereignty
Sarveśvara ('Lord of all,' from sarva = all + īśvara = Lord) is a title of Bhagavān Śrīman Nārāyaṇa affirming that His lordship (aiśvarya) admits no exception or boundary. Not a god among gods, but the God who is the inner self and sovereign of all other devas, all souls, and all matter.
Distinction from Other Devas
Viśiṣṭādvaita's insistence on Sarveśvaratva (the quality of being the Lord of all) is the theological basis for its refusal to treat Brahmā, Rudra, Indra, or any other deva as supreme. All devas are cit (souls) embedded in Brahman's body — however exalted, they remain śeṣas (dependents) of the one Sarveśvara.
In Devotional Usage
In the Āzhvārs' hymns, Sarveśvara is not a cold theological claim but a devotional exultation: 'My Lord is Lord of all!' — which makes the devotee's relationship with Him all the more astonishing ('such a Lord has become mine'). The contrast between His absolute sovereignty and His sauśīlya (gracious condescension) is a recurring theme of wonder in the Divya Prabandham.