General:
* Ceasefire
* Armistice
* Suspension of hostilities
* Peace (in a temporary sense)
* Agreement
* Lull
* Respite
* Break
* Intermission
More specific:
* Parley (a discussion to negotiate a truce)
* Treaty (a formal agreement, often ending a war)
* Compact (a formal agreement)
* Entente (an understanding or agreement between nations)
The best synonym to use will depend on the specific situation and the tone you want to convey.