A woman, usually of blonde follicles, whom is indistinguishable from her counterparts due to the fact that the back of her head is the only thing kept in the memory of a male victim seized at will after an evening of Mexican dining. This is usually the result of most of the evening resulting in an exercise of "knobbing" or being felatious. This can also result from hanging out is a doggy style arrangement or reverse cowgirl orientation.
Beware when the Zeta Knob approaches from the rear, this is when she is most dangerous. Lurking in the shadows and then quickly positioning her head into the felatio zone is what has kept this breed from dying out the past million years.