The novel Once an Eagle is divided into five 'books.' Each 'book' is titled by a single word, such as "Wheat" or "Chaparral," which serves to suggest the theme. These words/titles cannot properly be called epigraphs because they are not full quotations; what is the technical term for a one-word 'epigraph'?
The division that you're talking about is sometimes referred to as a part. Unless I'm mistaken, there is no more accurate (or "technical") term for it.
Typically books have chapters. Within each chapter can be scenes or sections.
A "part" is composed of multiple chapters—organized by some significant theme or event.
Parts can be labelled "Book One" or "Part One", or they can have more descriptive titles. They act as logical divisions.
In your case, these "part labels" are single words.
You are, however, correct that these single words would not typically be considered epigraphs. An epigraph is a phrase or quotation, and normally has an attributed source. While epigraphs can be used to mark divisions in books, I have never seen one that is only a single word.
Perhaps in the same way that we have chapter titles, what you're referring to could be called division titles. (I would say "part titles," but that doesn't sound right.)
I don't know if there is a universally accepted name for this or not.