Among the various heroic deeds of the god Indra extoled in the religious poetry of the R̥ gveda (ca. 1200 BCE), the foremost one is the battle with Vr̥tra ‘obstacle’, a gigantic cobra enclosing the waters. In this battle Indra kills Vr̥tra and releases the waters to the world. It is because of the prominence of this event ascribed to Indra in the R̥ gveda that the ancient etymologist Yāska (ca. 5th–4th c. BCE) declares ‘the slaying of Vr̥tra’ (vr̥ travadha) and ‘the giving of water (essence)’ (rasānupradāna) to be the main activities (karman) Indra performs (Nirukta 7.10). Yāska states elsewhere that etymologists (nairukta) understand the word vr̥ tra as a name for ‘cloud’ (Nirukta 2.16), which amply indicates that ‘the slaying of Vr̥tra’ and ‘the giving of water’ in question should mean ‘the opening of clouds’ and ‘the producing of rain’. Accordingly, Yāska first etymologizes the name indra by making use of the word irā ‘refreshment’ which is clearly intended to denote ‘rainwater’ as in R̥ gveda VII.65.4 (íḷā), thus characterizing Indra as a rain god (Nirukta 10.8). This type of etymological interpretation of the name indra is not found in Vedic literature before the Nirukta.
Yāska takes the accounts of the slaying of Vr̥tra and the release of the waters, provided in the R̥ gveda, as metaphorical expressions (upamā) of the natural phenomenon of rainfall (Nirukta 2.16). According to Yāska, rainfall takes place when water and light are mixed together (apāṃ ca jyotiṣaś ca miśrībhāvakarmaṇo varṣakarma jāyate). This explanation is evidently based on empirical observation of a natural phenomenon in which water starts falling as rain from clouds with occasional flashes of lightning. It is therefore reasonable to suggest that Yāska perceives the natural phenomenon of rainfall as consisting of three elements: clouds, (rain)water, and lightning. Among these, the former two correspond to Vr̥tra and the waters encompassed by it. Then, it is most natural to deduce that in Yāska’s theology lightning corresponds to Indra, who kills Vr̥tra and releases the waters; and hence this god turns out to occupy the same position as another god of lightning residing in the midspace region (antarikṣa): Jātavedas, one of the three fiery gods in Yāska’s system (Agni, Jātavedas, Vaiśvānara). Śaunaka (ca. 5th c. CE) even identifies Indra with Jātavedas in Br̥haddevatā 2.30.
In Yāska’s theology, Indra is most prominently a personification of lightning who dominates the midspace region (Nirukta 7.5; 7.10) and controls the rainfall that is indispensable for human life.