Does anyone know if there's a term that describes the device of titling chapters in a work of fiction? That is, chapters not simply called "Chapter 1", "Chapter 2", etc., but chapters with unique names that describe their contents, such as the chapter called "The Potions Master" in the first Harry Potter book.
It is a chapter whether it is titled or numbered; there's no term exclusively for a chapter with a title. If you really need to distinguish between them, you might call them titled chapters.
NOAD: a main division of a book, typically with a number or title.
Random House Dictionary (via Dictionary.com): a main division of a book, treatise, or the like, usually bearing a number or title.
Collins English Dictionary (via Dictionary.com): a division of a written work, esp a narrative, usually titled or numbered
The American Heritage Dictionary (via TheFreeDictionary.com): One of the main divisions of a relatively lengthy piece of writing, such as a book, that is usually numbered or titled.
I believe you are asking if there is a literary term for the use of named chapter titles, the way there is, for example, the term "unreliable narrator" for the use of a narrator whose statements cannot be trusted to be true within the world of the story.
As far as I know, there is no jargon that covers this, you would simply refer to using chapter titles. "J. K. Rowling used chapter titles to build suspense in the first Harry Potter book."