An allusion is a reference to commonly held cultural knowledge. In Act II, Scene 2, there is an allusion to Greek mythology in lines 170-172.
Else would I tear the cave where Echo lies,(170)
And make her airy tongue more hoarse than mine
With repetition of my Romeo's name.
Echo was a mountain nymph (thus the reference to "airy tongue") who loved the sound of her own voice. She fell in love with Narcissus, the vain young man. She followed him, repeating everything he said until she showed herself. When he spurned her, she was devastated. In one version of the myth, echo wandered until she found a cave to live in.