a linear earthwork that was tall enough to cover the chest of a man standing behind it. A breastwork could also be constructed of ad hoc materials, such as logs, stones, barrels, or cotton bales. The term is used very generically.
A breastwork is a fortification. The term is usually applied to temporary fortifications, often an earthwork thrown up to breast height to provide protection to defenders firing over it from a standing position. A more permanent structure, normally in stone, would be described as a parapet or the battlement of a castle wall.