An allusion is a literary device wherein an author invokes a well-known story or event from history as a point of reference or comparison. There are two allusions to mythic figures in Act II, Scene II of Shakespeare's Romeo and Juliet.
First, Juliet alludes to Greek/Roman mythology when she says, "at lovers' perjuries/They say, Jove laughs." Jove is another name for the Roman god Jupiter, also known as Zeus. Though the characters of Romeo and Juliet are Catholic, as evidenced by the presence of Friar Lawrence as a main character, Juliet invokes not the Christian God but an older god from legends to emphasize her point. This line specifically is a reference to a poem by the Ancient Roman poet Ovid which would have been well-known by the Elizabethan audiences of Shakespeare's time. The original quote from Ovid's Ars Amatoria: "Jupiter from on high smiles at the perjuries of lovers"
Later in the scene, Juliet again invokes Greek mythology by comparing herself to Echo, a nymph who, as her name suggests, was cursed by the goddess Hera to forever repeat the words that were spoken to her. Juliet's lines containing the allusion are as follows,
"Else would I tear the cave where Echo lies,
And make her airy tongue more hoarse than mine,
With repetition of my Romeo's name."