The opening pages of Yaska's Nirukta
In Nirukta, Yaska determines the etymological meaning of the term “Deva” (the modern equivalent would be God), by considering the roots da, dip, and dyut. In verse 7.15, he notes: “devo dānādvā dīpanādvā dyotanādvā dyusthāno bhavatītivā.” This means that Deva is the one who bestows gifts, who shines, who is bright, and who resides in heaven. In verse 7.2.5, he classifies the Gods mentioned in the Vedas into three divine entities: “tisraḥ eva devatā iti nairuktāḥ | agniḥ pṛthivīsthānaḥ vāyurvā indro vā antarikṣasthānaḥ, sūryo dyusthānaḥ” This means that the three Deva are: “Agni who resides on earth, Vayu or Indra who reside in the air, and Surya who resides in the sky and is always moving.” Yaska also says that whatever is venerated by a devotee in a hymn becomes a Deva.
It is believed that Panini, another great grammarian and linguist of the ancient age, came after Yaska.This is because Yaska’s contribution to etymology is mentioned in a sutra in Panini’s Aṣṭadhyayi. There were other grammarians who came before Yaska—for instance, there is Rishi Shakalya, whose work has been criticized by Yaska.