Belfredus, is used by military writers of the middle age for a fort or tower erected by besiegers to overlook and command the place besieged. Belfry originally denoted a high tower, whereon sentinels were placed to watch the avenues of a place, and prevent surprise from parties of the enemies, or to give notice of fires by ringing a bell. In the cities of Flanders, where there is no belfry on purpose, the tower of the chief church serves the same end. The word belfry is compounded of the Teutonic bell, and freid "peace," because the bells were hung for preserving the peace.

Belfry is also used for that part of a steeple wherein the bells are hung. This is sometimes called by middle-age writers campanile, cloacaria, and triftegum.

Belfry is more particularly used for the timberwork which sustains the bells in a steeple, or that wooden structure to which the bells in church-steeple are fastened.