Theravadin monk and scholar Walpola Rahula wrote,
“There is no term in Buddhist terminology wider than dhamma. It includes not only the conditioned things and states, but also the non-conditioned, the Absolute Nirvana. There is nothing in the universe or outside, good or bad, conditioned or non-conditioned, relative or absolute, which is not included in this term.” [What the Buddha Taught (Grove Press, 1974), p. 58]
Dhamma is the nature of what-is; the truth of what the Buddha taught. In Theravada Buddhism, as in the quote above, it is sometimes used to indicate all the factors of existence.
Thanissaro Bhikkhu wrote that “Dhamma, on the external level, refers to the path of practice the Buddha taught to this followers” This dhamma has three levels of meaning: the words of the Buddha, the practice of his teaching, and the attainment of enlightenment. So, dhamma is not just doctrines; it is teaching plus practice plus enlightenment.
The late Buddhadasa Bhikkhu taught that the word dhamma has a fourfold meaning. Dhamma takes in the phenomenal world as it is; the laws of nature; the duties to be performed in accordance with the laws of nature; and the results of fulfilling such duties. This aligns with the way dharma/dhamma was understood in the Vedas.
Buddhadasa also taught that dhamma has six attributes. First, it was taught comprehensively by the Buddha. Second, all of us can realize dhamma through our own efforts. Third, it is timeless and present in every immediate moment. Fourth, it is open to verification and does not have to be accepted on faith. Fifth, it allows us o enter Nirvana. And sixth, it is known only through personal, intuitive insight.
— from “Dhamma Meaning in Buddhism” – About.com