A bilateral treaty (also called a bipartite treaty) is a treaty strictly between two state parties. These two parties can be two states, or two international organizations, or one state and one international organization.
It is similar to a contract, so it is called contractual treaty.
See also 
- A Brief Primer on International Law With cases and commentary. Nathaniel Burney, 2007.
- Official United Nations website
- Official UN website on International Law
- Official website of the International Court of Justice