Before the publication of the texts, when they were known in the West only by hearsay, the term Abhidhamma was usually rendered Metaphysics. This is now seen to be quite erroneous. Dhamma means "the doctrine" (see Wikipedia entry on dharma for alternative translations) , and Abhidhamma has a relation to Dhamma similar to that of by-law to law. It expands, classifies, tabulates, draws corollaries from the ethical doctrines laid down in the more popular treatises. There is no metaphysics in it at all, only psychological ethics of a peculiarly dry and scholastic kind.
As in the course of centuries the doctrine itself, in certain schools, varied, it was felt necessary to rewrite these secondary works. This was first done, so far as is at present known, by the Sarvastivadins ("Realists"), who in the century before and after Christ produced a fresh set of seven Abhidhamma books. These are lost in India, but still exist in Chinese translations. The translations have been analysed in a masterly way by Professor Takakusu in the article mentioned below, they deal only with psychological ethics. In the course of further centuries these books in turn were superseded by new treatises; and in one school at least, that of the Mahayana, there was eventually developed a system of metaphysics. But the word Abhidhamma then fell out of use in that school, though it is still used in the schools that continue to follow the original seven books.
See Buddhist Psychology by Caroline Rhys Davids (London, 1900), translation of the Dhamma Sangani, with valuable introduction; or the Royal Asiatic Society, 1892, contains an abstract of the Katha On the Abhidhamma books of the Sarvastivadins, by Prof. Takakusu, in Journal of the Pali Text Society, 1905.
(l'. W. R. D.)