Proper noun
A group of four of Shakespeare’s plays: Richard II; Henry IV, Part 1; Henry IV, Part 2; and Henry V, with the implication that these four plays are Shakespeare’s epic poem.
Either of two groups of four each of Shakespeare’s plays: The "first Henriad" (Henry VI, part 1, Henry VI, part 2, Henry VI, part 3, and Richard III), and the "second Henriad" (Richard II; Henry IV, Part 1; Henry IV, Part 2; and Henry V).[1]
The title translated into English of an epic poem, Le Henriade, by Voltaire.
Source: en.wiktionary.org