What is the name of the physical feature that is the longitudinal valley lying between the Lesser Himalayas and the Siwalik in India?

The longitudinal valleys formed between the Lesser Himalayas (Himachal) and the outermost Himalayas (Siwaliks) are physiographically known as Duns in the west and Duars in the east. Dehradun, Kotli Dun, and Patli Dun are prominent examples of these flat-bottomed structural valleys.
Let's look at the other geographical terms:

* Doab: A region lying between two confluent rivers.
* Bhabhar: A narrow porous belt of pebbles deposited along the foothills of the Siwaliks.
* Terai: A marshy, damp, and thickly forested region lying south of the Bhabhar belt.