Step 1: Definition of Adipurana.
Adipurana translates to "the first Purana" and is recognized as the oldest of the 18 Mahapuranas.
Step 2: Identification of Brahmapurana.
Brahmapurana is designated as the Adipurana due to its position at the commencement of the Mahapuranas list.
Its content encompasses cosmology, creation narratives, genealogies, and accounts related to Lord Vishnu and Lord Shiva.
Step 3: Non-Adipurana Texts.
- Shivapuranam: Devoted to Lord Shiva, it is not identified as Adipurana.
- Agnipurana: Covers a broad spectrum of topics but is not classified as Adipurana.
- Vishnupuranam: A significant Purana, yet it is not referred to as Adipurana.
Step 4: Final Determination.
Consequently, the Purana known as Adipurana is the Brahmapurana.