The smallest part of a physical substance which cannot be divided further, is called aṇu (and sometimes, paramāṇu). According to the Vaiśeṣika system of philosophy, the four elements viz., earth, water, air and fire are atomic in size. The perceptible elements are formed out of a combination of these atoms.
The Viśiṣṭādvaita Vedānta and the Pāśupata system of Śaivism consider the ātman (the individual soul) as aṇu or atomic in size.
The word is sometimes used to indicate a minute unit of time also: 54,675,000th part of a muhūrta (of 48 minutes). In prosody it is the fourth part of a mātrā (syllabic instant).