Mahajanapadas literally means "Great Kingdoms".The political structure of the ancient Indians appears to have started with semi-nomadic tribal units called JanaThis process of first settlement on land had completed its final stage prior to the times of Buddha and Panini. The Pre-Buddhist North-west region of Indian sub-continent was divided into several Janapadas demarcated from each other by boundaries.
The Buddhist and other texts only incidentally refer to sixteen great nations (Solasa Mahajanapadas) which were in existence before the time of Buddha.
The Buddhist Anguttara Nikaya, at several places, gives a list of sixteen great nations:

Kasi
Kosala
Anga
Magadha
Vajji (or Vriji)
Malla
Chedi
Vatsa (or Vamsa)
Kuru
Panchala
Machcha (or Matsya)
Surasena
Assaka
Avanti
Gandhara
Kamboja