Etymology of Doab Region (Hindustani Punjabi Praant)
The word Doab is a combination of the Indo-Iranian words Doab (Two) and āb (Water), and thus the (Land of) Two Rivers. The two rivers are the Beas River and Sutlej River. Sometimes, in English, there can be a definite article before the name i.e. the Doaba. The name is also sometimes spelled as Duoab or Douaab or Doaab.
It originally contained 5 rivers before the formal “Partition of the Punjab”, and Independences of India and Pakistan respectively, British influence and the influence of the Indian federal Government, so therefore its name came from the Punjabi word for five (Panj/Punj) and the word for river/water (ab).
In the Oxford Hindi-English Dictionary, R. S. McGregor defines it as from Persian do-āb (دوآب, literally "two [bodies] of water") "a region lying between and reaching to the confluence of two rivers (esp. that between the Sutlej and Beas)."