A word, phrase or image that returns several times throughout the work, tied to a character or an idea, and gains meaning with each return.
In Romeo and Juliet
Is it e'en so? Then I defy you, stars!
The "stars" had already appeared in the Prologue and in Romeo's speech on the way to the ball, and now here. The return of the image links the last act to the earlier ones and makes fate a constant theme of the play.