Definition of treaty by Cambridge Dictionary Online:
a written agreement between two or more countries, formally approved and signed by their leaders
Definition of pact by Cambridge Dictionary Online:
a formal agreement between two people or groups of people
So should I call it a Non-aggression Treaty or Non-aggression Pact? They seem to be the same for me when is used.
Some sources on the Internet suggest that a treaty is between countries while a pact is between alliances.
e.g. Nazi-Soviet Pact
Source: https://answers.yahoo.com/question/index?qid=20090510153115AABqJ4A
However, I find that it is inaccurate because the Cambridge Dictionary Online gives this example sentence:
The United States and Canada have signed a free-trade pact.
Source: http://dictionary.cambridge.org/dictionary/english/pact
I also remembered there is a source that says Non-aggression Pact is to end current wars while Non-aggression Treaty is just an agreement.
Best Answer
There are no exact synonyms expect where stipulated (say in the sciences).
But treaty and pact are vague enough and overlap in meaning enough that they can easily be swapped in many contexts.
Did I mention that dictionaries aren't perfect either. That is constraint of dictionary writing, one may have limits on the length of a definition and nuances one can state (even the OED has some constraints, though fewer than most).
A treaty is an agreement between nations, either enemies or allies. It is written and authorized and ratified by both/all sides. Sometimes for variation, it might be nice to call this a pact between the nations.
A pact is a little more loose. I can make a pact with a friend of mine to always meet on New Years Day. But to call that a treaty would be wrong. If I made a similar arrangement with a person who is not my friend, say the owner of a competing business to informally discuss avoiding encroaching on the others business, that might be metaphorically called a treaty, but pact would work better.