Abhidhamma is a particular approach to the Dhamma which began to develop during the Buddha's time and reached its final form some two or three centuries after his passing. The word abhidhamma means `pertaining to dhammas.' Abhidhamma developed when early Buddhist thinkers tried to analyse phenomena into their most basic units which they called dhammas and then attempted to describe their characteristics, their duration, their interaction with one another and their kammic results. Whereas the Buddha generally restricted himself to empirical experience, abhidhamma tends to be more speculative.
Abhidhamma Studies, Nyanaponika, 1985.