The Pāḷi term Abhidhamma is composed of "Abhi" and "dhamma". "Abhi" means higher, subtle or ultimate and "dhamma" means truth or teaching. So, Abhidhamma is the higher or subtle teaching (of the Buddha) or the ultimate truth.

Abhidhamma is the Buddhist philosophy, describing the reality and truth. Abidhamma is the Buddhist psychology, dealing mainly with mental phenomena and explaining how our mind works.

Abhidhamma is the third great division of the Pāḷicanon or Tipiṭaka (three baskets). It is a huge collection of systematically arranged, tabulated and classified teachings of the Buddha, representing the quintessence of his teachings, timeless and independent of culture, race and gender.

In the Suttas (discourses) the Buddha takes into consideration the intellectual level of his audience, their development of the pāramīs (perfections), their attainments and the special situation. He therefore teaches the Dhamma in conventional terms and relative concepts (paññatti), making references to persons and objects as I, we, he, she, man, woman, cow, tree etc.
In the Abhidhamma, however, the Buddha does not make such concessions, but is treating the Dhamma entirely in terms of the ultimate reality (paramattha). All phenomena are analysed into their ultimate constituents (dhammas) which are precisely defined, classified and systematically arranged. Then the laws of interaction between the dhammas are taught, their synthesis - a net of conditionality. Anattā, the Buddha’s most important theory of non-self, can be fully understood by the Abhidhamma.