Such collections share characteristics with glossaries and thesauri, but are not true lexicons, such as the kośa of Sanskrit literature.
While a number of nighantavas devoted to specialized subjects exist, the eponymous Nighantu of the genre is an ancient collection, handed down from Vedic times.
According to Yaska (Nirukta 1.20), the Nighantu was a collection of rare or difficult words gathered by earlier sages for easier understanding of Vedic texts that perhaps they may not have fully understood themselves.
The collection comprises five adhyāyas or chapters, in three kāṇḍas or books: Yaska's Nirukta covers the Naighantuka somewhat synoptically, often merely echoing a given gloss, but deals with the 278 words of the Naigama and the 151 names of the Daivata in detail.
In it, two major recensions were identified, one longer than the other, indicating additions at untraceable yet relatively early dates.