I, 2, 84.
In a special theatrical sense, the prompter denotes a person stationed out of sight of the audience to prompt or assist any actor who is at a loss in remembering his part. In early days of the drama, the usual word for this official was book-holder, and is so quoted in Higgins’ Junius Nomenclator, 1588: “He that telleth the players their part when they are out and have forgotten. The prompter or book-holder.”
Ben Jonson uses the word book-holder in several of his plays, likewise many dramatists of this period. The word is now obsolete.
Other references:
PROLOGUE.
Is he often thus?
’Tis ever more the prologue to his sleep.
II, 3, 134.
An index and obscure prologue.
II, 1, 264.