The Nighatus are the glossaries or lists or rare and obscure words occurring in the Vedic hymns.
According to Yaska they were collected and classified by the descendants of ancient sages for the easier understanding of the Vedic texts. The Nirukta is a famous work of Yaska. It is the oldest Indian Treatise on Etymology, Philology and Semantics.
This being a commentary on the Nighantus collection of obscure words which tradition ascribes also to Yaska, follows the three-fold division of the contents of the Nighantus.