1) Like "Oedipus Complex", except for less creepy and far sexier
2) Being severely sexually attracted to your girlfriend's or wife's mother, as described in the Fountains of Wayne hit "Stacy's Mom."
3) Willingness to leave your significant other for his/her mother.
Guy One- "Dude, the only woman I would leave my girlfriend for is her mom. Have you seen her?!?!?"
Guy Two- "Yeah man, your Stacy's Mom Complex is completely justified"