The Atomic Soul
Aṇu means 'atom,' 'infinitesimally small.' In Vishishtadvaita philosophy, the jīva is said to be aṇu in its essential nature (svarūpa) — its intrinsic self has no spatial extension. This is to be distinguished from the vibhu (all-pervading) Brahman.
Scriptural Basis
The Muṇḍakopaniṣad: eṣo'ṇur ātmā cetasā veditavyo — 'this atomic self is to be known through the mind.' The Chāndogya's aṇor aṇīyān also reflects this — the self is subtler than the subtle. Viṣṇu Purāṇa and Pañcarātra texts elaborate technically on the distinction between the aṇu-jīva and vibhu-brahman.
Dharma-bhūta-jñāna Expands
Crucially, the jīva's being aṇu in svarūpa does not limit its consciousness. Vishishtadvaita holds that the jīva's dharma-bhūta-jñāna (attribute-consciousness) is a separate category that can expand like light to illuminate objects far from the essential self. In bondage, this knowledge is contracted by karma; in liberation, it expands to experience Brahman fully.
Distinction from Advaita
Advaita holds that the jīva is ultimately identical to the infinite Brahman (tat tvam asi), with apparent smallness being illusion. Vishishtadvaita rejects this: the jīva is genuinely aṇu and genuinely distinct — though not separate — from vibhu Brahman. Individual selfhood is never lost, even in mokṣa.