Svarūpa Lakṣaṇa — The Intrinsic Defining Character
Svarūpa Lakṣaṇa (Sanskrit: sva = own + rūpa = form/nature + lakṣaṇa = characteristic; 'own-nature characteristic') is the essential, intrinsic defining quality of an entity — a characteristic so fundamental that its absence would mean the entity itself ceases to exist or be what it is.
In Contrast to Tāṭastha Lakṣaṇa: The paired concept tāṭastha lakṣaṇa is an accidental or contingent characteristic — one that may or may not be present without affecting the entity's fundamental nature. For example, the svarūpa lakṣaṇa of Brahman is sat-cit-ānanda (existence-consciousness-bliss); these can never be absent. The tāṭastha lakṣaṇa of Brahman is 'the cause of the universe' — contingent on creation occurring.
Application to the Ātman: The svarūpa lakṣaṇa of the ātman is jñāna (consciousness) and ānanda (bliss). These are inseparable from the ātman. In saṃsāra these are obscured, but in liberation they shine in their full natural glory. The ātman does not 'become' blissful — bliss is its svarūpa.
Theological Importance: Understanding svarūpa lakṣaṇa vs. tāṭastha lakṣaṇa helps avoid a fundamental error: treating contingent attributes as essential, or essential nature as contingent. The ātman's bondage is tāṭastha (contingent, removable); the ātman's śeṣatva (servant-hood to Bhagavān) is svarūpa (essential, permanent).