The Cambridge History of India, Vol. I: Ancient India v1.c19.s9.4

v1.c19.s9.4

So far the books had been divided into Dhamma and Vinaya; that is to say, religion and the regulations of the Order. Now, after the close of the canon, a new division begins to appear, that into three Piṭakas (or Baskets) of Vinaya, Sutta, and Abhidhamma. We do not yet know exactly when or why this new division arose and superseded the older one². As late as the fifth cent. A.D. we find Buddhaghosha still putting the Vinaya and the Abhidhamma into the supplementary fifth Nikāya¹, though he and other commentators also use the newer phrase².