The curious tendency of male adolescents to be more attracted to a girl's mom over the girl herself. It is completely understandable if there is a consensus among peers supporting that the mom is indeed unmistakably hot.
Guy1: Dude, did you see Stacey yesterday? So hot.
Guy2: Naww, man. Check out her mom.
Guy1: Hell yes. Stacey's Mom Syndrome my friend.