/structure.

JA:  Now, if you really think about it, a table is like a concept,
     namely, a concept of some relation that you find to be of use.
     Think about the way a medical research database changes when
     somebody hypothesizes a new disease entity, starts to keep
     tabs on its signs, symptoms, test results, mortality data,
     and then either confirms or discards the notion.

DW:  Some of those fields/columns/data-elements represent
     concepts or groups of concepts.  Some do not.

JA:  