When a certain person in a relationship sees someone he or she is mentally or physically attracted to other then his significant other.
Josh was dating Emily, but he had a huge wandering eye for the new girl at school.
A penis: A man's penis has one hole resembling an eye in which it wanders from person to person searching or looking for the one in which it chooses!
That man's wandering eye was satisfied by that cute girl from the bar last night.
Not to be confused with a whispering eye, as in Role Models, a wandering eye is a vagina that has moved from its original place (a prolapse). It most commonly happens with women that have just had children or with old ladies that are severely disappointed with their grandchildren and have already banished half of them from the family.
Jack: Do you want to go have sex in the garden tonight?
Gemma: No, sorry I can't I have to go visit my Grandma in the hospital.
Jack: What's wrong with her?
Gemma: The extreme hatred of her grandchildren gave her a wandering eye.