We now have our basic definitions, but we need to put some size restrictions on them, else everything is some type of structure. For instance, the main lake body could be seen as a hole, or simply a round set of ledges. We need to decide which one because we'd like to fish the structure differently depending on which type of structure we call it. As a reasonable guide, let's look for just a second on how fish use structure. Structure, what ever it's form, is a type of underwater transportation system.
Fish generally follow a narrow depth getting from one place to another.
The advantage of using structure edges is the deeper water provides an immeadiate excape route if you happen onto something much bigger than you. Now just like any road, fish rarely live right in the center lane. More likely, they will be just off one side or the other, which is were the cover part comes in. In any event, for our purposes, the extent or limit of a useful definition for a structure is the depth change itself and the reasonable area adjacent to the drop where fish will wander off to feed. In the case of our hole, a fish might reasonably wander out, say 100 feet into the hole without loosing the advantage of the edge for chasing shad.