Purāṇa — Narrative Memory
The first three upāṅgas protect and interpret the Veda. The fourth makes it reachable. Itihāsa-purāṇa is the Veda told as story — and the tradition calls it the fifth Veda for exactly that reason.
Five marks of a Purāṇa
A text qualifies as a Purāṇa by its five themes: creation (sarga), dissolution and renewal (pratisarga), the ages of the Manus (manvantara), the lineages of the gods and seers (vaṃśa), and the histories of the solar and lunar houses (vaṃśānucarita).
The corpus
Eighteen Mahāpurāṇas and eighteen Upapurāṇas, ordered by deity — Brahma, Vaiṣṇava, Śaiva — and supplemented endlessly by Sthalapurāṇas rooted in particular places and pilgrimages. In them, philosophy, ritual, geography, and devotion are held together in narrative rather than in argument.
This is not the Veda diluted. It is the Veda transported — into the kitchen, the temple, and the festival.