(n.) The ability to sense when a person of the opposite sex is flirting with you. Females have been known to have better fradar than males, and so the term derived from women.
Girl, my fradar is going off on that sexy man.
Oh hayyyy, my fradar be pickin up on yo' teasin'!
Friend-radar, a person or group of people that you wish to become friends with.
Chris is definitley on my fradar, he's so legit!