Formal Agreements:
* Pact: A formal agreement, especially one between nations.
* Convention: A formal agreement between states or nations.
* Covenant: A formal agreement, often with a religious or moral basis.
* Protocol: A set of rules or procedures, often related to a treaty.
* Concordat: A formal agreement, especially between the Church and a state.
* Compact: A formal agreement between parties.
Informal Agreements:
* Accord: A state of agreement or harmony.
* Understanding: A mutual comprehension of something.
* Arrangement: An agreement or plan.
Other related terms:
* Ratification: The formal approval of a treaty.
* Negotiation: The process of discussing and agreeing on the terms of a treaty.
To choose the most appropriate synonym, consider the specific type of treaty and its context.