A woman who, whether intentionally or not, makes you consider cheating on your wife.
She is often your wife's younger, sexier sister or friend, that you flirt with because you really want to screw her brains out. However she could just be a random slut you encounter in a bar, shopping mall, or on the street. Affair bait typically does things that your wife doesn't, like give head and have anal sex.
My wife's friend just texted me, saying she's super horny, drunk, and lonely ... God she's total affair bait. Too bad I'm stuck at home watching 20/20 on television with my lame wife.