A ceasefire (or truce), also spelled cease fire (the antonym of 'open fire'), is a temporary stoppage of a war in which each side agrees with the other to suspend aggressive actions. Historically, the concept existed at least by the time of the Middle Ages, when it was known as a 'truce of God'. Ceasefires can be declared as a humanitarian gesture, be preliminary, i.e., prior to a political agreement, or definitive, i.e., with the intention of resolving a conflict. Ceasefires may be declared as part of a formal treaty, but they have also been called as part of an informal understanding between opposing forces.