Nirukta is the science of word derivation and interpretation — understanding Vedic words by tracing them to their roots. Without Nirukta, the deep meanings of Vedic hymns remain inaccessible.
Yaska's Nirukta (~5th century BCE) is the foundational text. It systematically derives the meanings of difficult Vedic words, arguing that all nouns ultimately derive from verbal roots (dhatus). Yaska also categorizes words and provides multiple interpretive approaches for Vedic hymns.
The Nighantu is a collection of Vedic word lists organized by topic — the earliest known Indian glossary. Yaska's Nirukta serves as a commentary on the Nighantu, explaining the meanings and derivations of each listed word.
A special section of the Nirukta dealing with the names and natures of the Vedic deities. It derives the names of Agni, Indra, Vayu, Surya, and others from verbal roots, revealing the philosophical meanings embedded in their names.