An air route or flight operating within a country between domestic cities; domestic flight or route (or, by extension, a domestic carrier).
北海道へは国内線で2時間ほどかかる。
It takes about two hours by domestic flight to Hokkaido.
国内線の搭乗口が変更になったので案内板を確認してください。
The domestic boarding gate has changed, so please check the information boards.
格安の国内線プロモーションで出張費を節約できた。
I was able to save on business travel costs thanks to a discount domestic flight promotion.