What is Cynthia's brow?
In Scene 3, act 5 of Romeo and Juliet, Romeo is leaving Juliet, when she argues that it is a long time until daybreak and that they are hearing the nightingale (a night bird) rather than a lark (which heralds dawn). During night hours, it is safe for them to be together, but Romeo must leave before daybreak. Romeo says that he hears the lark, but then he changes his mind and decides that he should stay and risk capture. He then begins to argue that it is still night, and he says:
"I’ll say yon grey is not the morning’s eye. /'Tis but the pale reflex of Cynthia’s brow. /Nor that is not the lark, whose notes do beat /The vaulty heaven so high above our heads. /I have more care to stay than will to go" (III.5.19-23).
In other words, he says, that the grey light peaking over the horizon is not morning but the reflection of light coming from "Cynthia's brow." Cynthia is the goddess of the moon, as Cynthia is a Greek nickname for Artemis (the Roman goddess Diana), so Romeo says the light is a reflection of the moon, not the sun. Both images, the morning's eye and Cynthia's brow, turn the sun and moon into faces. He also argues that he does not hear the lark, contradicting what he said earlier, and he wants to stay rather than to leave Juliet. These contradictions between night and day, sun and moon, support the idea that Romeo and Juliet's love is the opposite of the way it should be. In warring Verona, they must hide by the light of the moon rather than emerge in the sunlight.
I believe it is a reference to the Goddess Artemis, who was also sometimes called Cynthia. So, when Romeo says:
I'll say yon grey is not the morning's eye,
'Tis but the pale reflex of Cynthia's brow.
Nor that is not the lark, whose notes do beat
The vaulty heaven so high above our heads:
I have more care to stay than will to go:
He is expressing the fact that parting is so impossible that he will not even recognize common things such as morning and birds, but instead will see Goddesses, such is the power of love over reality.
Also of note is that Cynthia (Artemis) has been described as "both the virgin huntress and the maternal protectress of wild, young things", which you could describe Romeo and Juliet as. Shakespeare has used her name and image before in the Two Gentlemen From Verona.