A recent chatroom discussion about how to tag the question Portrayal of Henry Bolingbroke through different Shakespeare plays led to the question what "Henriad" actually means.
According to Wikipedia, Shakespeare scholars have used the term to refer to the sequence Richard II, Henry IV, Part 1, Henry IV, Part 2 and Henry V. (These plays were also written in a chronological order that matches the order of the historical events they are inspired by.)
Some scholars have also included the plays inspired by the War of the Roses: Henry VI, Part 1, Henry VI, Part 2, Henry VI, Part 3 and Richard III. (In this sequence, the play now known as Henry VI, Part 1 was written after the other Henry VI plays.) Since this "tetralogy" was written before the one mentioned above, it is also referred to as the "first tetralogy", while the sequence starting with Richard II is referred to as the "second tetralogy".
"Henriad" can refer to just the "second tetralogy" or can include all of the above plays, except when the term "second Henriad" is used, i.e. as a synonym for "second tetralogy".
Wikipedia tells us that Alvin Kernan "popularized" the term in a article published in 1969. The term sounds like an anglicisation of Henriade, the title of a poem about Henry IV of France (not England) written by Voltaire in 1723. Since popularising a term is not the same thing as coming up with a term, it is not entirely clear whether Kernan was the first scholar to use the term "Henriad" to refer to the second or first tetralogy.
So who was the first scholar to use the term "Henriad" as described in Wikipedia and how was the term originally defined? Was it Kernan or somebody else?