What is described as a bulge or the protuberance of a cask?

The term that defines a bulge or protuberance of a cask is "bilge." This word is specifically used in the context of shipbuilding and vessels, where it describes the lowest part of the hull, which often swells or bulges due to the collection of water and other materials. In relation to casks, the bilge refers to the bulging part of the cask's structure that can occur as a result of the shape or contents. It captures not just a physical descriptor but also evokes the idea of storage and containment found in traditional cask design.

The other terms do not capture this specific meaning. “Girth” refers to the measurement around the center of an object, often used in a more general sense concerning dimensions rather than describing a bulge. “Curve” denotes a bend or arc, which lacks the specific implication of protrusion associated with "bilge." Lastly, “expansion” commonly refers to an increase in size or volume without any connotation of a bulging form. Hence, bilge is the precise term that describes the protruding aspect of a cask.