principle of reciprocity
Used in legal, diplomatic, and economic contexts to describe a policy where one party's actions toward another are mirrored. Often appears in phrases like 相互主義に基づく (based on the principle of reciprocity).
The treaty was concluded based on the principle of reciprocity.
From the standpoint of reciprocity, we request similar measures from the other country.
bilateralism
Compound of 相互 (mutual, reciprocal) and 主義 (principle, doctrine). The term is a direct translation of the Western concept of reciprocity in international relations and law.