The main road or highway; figuratively, the right or proper way.
この先は本道だから交通量が多い。
This is the main road ahead, so traffic is heavy.
道に迷ったら、本道に戻ると分かりやすい。
If you get lost, it's easy to find your way by returning to the main road.
議論の本道に戻って、結論を出そう。
Let's return to the main point of the discussion and reach a conclusion.
In traditional Chinese medicine contexts, a term sometimes used to refer to internal medicine (the internal department).
古い診療記録には本道（＝内科）と外科の区別がある。
Old medical records distinguish between internal medicine (hondō) and surgery.
その医家は本道に詳しく、体調不良の治療を得意としていた。
That physician was knowledgeable in internal medicine and specialized in treating general ailments.