General:
* Ceasefire
* Armistice
* Agreement
* Suspension of hostilities
* Temporary peace
* Stalemate (implies a standstill in fighting, not necessarily a formal agreement)
More formal:
* Convention
* Treaty (especially for a longer-term agreement)
Informal:
* Time-out
* Break
* Cooling-off period
Specific types of truces:
* White flag (a signal of surrender or request for a truce)
* Safe passage (temporary protection for movement in a conflict zone)
The best word to use will depend on the specific situation and the desired level of formality.