One of the one hundred appellations of Viṣṇu (Vishnu). The term aṇu (anu) suggests molecule or molecular nature of a thing. Upaniṣadas (Upanishada), while describing the nature of brahma, has countless times referred to Him as aṇuparimāṇa (anuparimana) or ‘molecular in size’ and even ‘smaller than a molecule’ —
Muṇḍakopaniṣada (Mundakopanishada) observes that brahma is aṇuparimāṇa and must be realised with one’s conscience and soul —
eṣo’ṇurātmā cetasā veditavyo yasmin prāṇaḥ pañcadhā saṃviveśa.
As Viṣṇu, one of the pillars of the trinity of the Hindu pantheon, is often equated with this all-pervasive brahma, he also is hailed by this appellation of Aṇu —
saukṣmyātiśayaśālitvāt aṇuḥ.[Viṣṇusahasranāmastotra and its commentary by Śaṅkarācārya]