Definition, Origin, and Extent of "Matter" as a title in Literature

Why was "Matter" chosen for the Matter of Britain, the Matter of France and the Matter of Rome?

What would the exact definition of "Matter" be in this instance? Written/Printed Material, or a Theme/Group of Themes, or (obsolete) the essence; the pith; the embodiment? Whereas it seems to refer to topics, we often hear it used as a proper noun to describe the Literature itself, such as "in the Matter of Britain we often read of..." So what did it mean in its original context and what has it come to mean - exactly - in the English language?

Who coined this term and when? Was it the old French poet Jean Bodel (c. 1165 – c. 1210) who classified "The Three Matters" ["N'en sont que trois materes a nul home entendant / De France, et de Bretaigne, et de Rome la grant."], or was the term commonly used in regards to bodies of literature before him? And when if ever did it's use fall out of vogue?

The phrases "the Matter of Britain," "the Matter of France," and the "Matter of Rome" seem to have been taken directly into English from Jean Bodel's verse some time in the late 19th century. Before then, "the Matter of Britian" seems to have been called by other names, such as "the Arthurian Cycle," in English.

The first time I can find "Matter of Britain" used in Google books is in The Flourishing of Romance and the Rise of Allegory by George Saintbury, which was first published in 1897.

More evidence that these phrases were taken directly from Jean Bodel's verse into late 19th century English, and were not timeworn English phrases, comes from the 1920 book A History of English Literature, by William Allan Neilson, in which we find

The great majority of the romances fall into groups or cycles concerned with what an old French poet called the “matter of Britain,” the “matter of France,” and the "matter of Rome."

Since the word comes to us from Old French, and seems to have been translated into English using the cognate word matter (whether or not this was the best English word), what we really need is a good Old French dictionary.

The Anglo-Norman Dictionary has for one of the definitions of matire:

subject matter, topic (of a literary work),

which is exactly the sense that Jean Bodel used it in. (And matire is the same word as Jean Bodel's matere – Old French spelling was as variable as Middle English spelling.)

The OED has the very similar definition (which is not surprising, since a large amount of Old French vocabulary was incorporated into Middle English after the Norman Conquest):

"The subject of a book, speech, etc.; a theme, a topic, a subject of exposition. In early use also: a narrative, an account, a tale. Obsolete."

One of their citations for this meaning is (ca. 1340)

The matere of this boke is crist & his spouse.

It's a straight borrowing from the Latin Res "matter, thing," which we still use daily in the form of Re: "in the matter of:"; Latin books were often titled beginning De Re .... (Concerning the matter/nature of....), e.g. Agricola's De Re Metallica (On the Nature of Metals), Cicero's De Re Publica (On the Republic, which is literally the 'public matter') -- as distinct from the usage of genitive rerum as actual "things" in Lucretius's De Rerum Natura (On the Nature of Things).

Cf. Arthurus (rex): "Saeculis XI, XII, XIII multae mythistoriae de vita Arthuri scriptae sunt in Cambria, Francia, Anglia; id genus fabularum poetae Francici Matière de Bretagne appellabant, scilicet 'Res Britanniae'. "