The Settled Position
Siddhānta ('established conclusion,' from siddha = established + anta = end/conclusion) is the definitive doctrinal position that emerges after weighing all pūrva-pakṣa (opposing views) and responding to them. In Viśiṣṭādvaita, the siddhānta is the teaching of Rāmānuja and his commentarial tradition as the correct interpretation of Vedic, Smṛti, and Āgama scripture.
The Siddhānta of Viśiṣṭādvaita
The core Viśiṣṭādvaita siddhānta holds: (1) Brahman is Śrīman Nārāyaṇa, the supreme personal God; (2) cit (souls) and acit (matter) are real and distinct but form His body and are His modes; (3) liberation is the soul's experience of its true nature as Brahman's servant; (4) the valid means (upāya) to liberation is either bhakti-yoga or prapatti, both ultimately dependent on Bhagavān's grace.
Usage in Teaching
Teachers begin by presenting what appears to be the siddhānta, introduce objections (pūrva-pakṣa), and then restate the siddhānta with greater precision. This dialectical movement — thesis, objection, stronger thesis — is the engine of commentarial elaboration in the tradition.