I know this could potentially be a huge question: if abhidhamma/abhidharma differs in Theravada and Mahayana, how does is it differ and why?
There have been over 18 different schools within Buddhism, differentiating on the philosophical issues. Of those 18 schools, only two survive till the present day. The two are known as Mahayana and Theravada. Each of them have more sub-schools within them.
Abhidhamma [pali] or Abhidharma [sanskrit] is the name of one of the baskets (pitaka[sanskrit]) of the tri-pitaka- three baskets of teachings. As such, it accounts for the theorization and philosophical discussions of Buddha's teachings. Both Theravada and Mahayana rely on, take teachings of and consider abhidhamma as very important.
The differences occur not on the abhidhamma text itself but it's philosophical import- both hermeneutically and consequentially. As one very broad and simplified example, both schools whose source of philosophical discussion is abhidharma, take the concept of sunyata or emptiness. While Theravada focuses particularly on the emptiness of self and take that as one of the core principles, Mahayana instead focuses on emptiness of phenomenon wherein it then focuses on emptiness of self.
There are quite a reasons for the differences to occur. I will mention one. Theravada developed in Sri Lanka as the monks went there and while Mahayana developed in India it went via Tibet to east asia. As such, the sort of debates or discussion the monks had to engage in were culturally different and the responses were shaped accordingly. In that sense, Mahayana came after Theravada as a Buddhist response to the Non-Buddhist philosophical positions while Theravada didnt feel the need to respond this way because there was no philosophical challenges posed. Hence, both schools gradually shaped their understanding of the same teachings of Abhidharma.
Hope you find this of use to think more on the differences.
- Main difference is the Theravada Abhidhamma rejects the "Self" in whatsoever form.
- The "emptiness" advocated in the two doctrines are different in meaning.
- According to Theravada There are only 5 masses(/12faculties/18 elements) in this whole world.
- Theravada says that there is no any "Permanent entity" or "Comfortable entity" or "Doer entity" in this world.
- They analyze all the world in to elements and at the end no "person/doer" can be found.
- This is the main difference between Theravada and "not only Mahayana but also all the other non-theravada doctrines".
- The "emptiness" mentioned in Theravada is "emptiness of a self entity". Mahayana emptiness is very different from this.