Wikipedia
A Mahājanapada (Sanskrit महाजनपद) (literally "great realm", from maha, "great", and janapada "foothold of a tribe", "country") is one of the sixteen kingdoms or oligarchic republics that existed in the ancient India from the sixth centuries BCE to fourth centuries BCE. Two of them were most probably 'ganas' ie. republics, and others had forms of monarchy. Ancient Buddhist texts like the Anguttara Nikaya make frequent reference to sixteen great kingdoms and republics which had evolved and flourished in a belt stretching from Gandhara in the northwest to Anga in the eastern part of the Indian subcontinent and included parts of the trans- Vindhyan region, prior to the rise of Buddhism in India.
The 6th century BCE is often regarded as a major turning point in early Indian history. Archaeologically, this period corresponds in part to the Northern Black Polished Ware culture.