(n) - An unstable combination of women that increases ambient bitchiness. This can occur when the natural vibrating frequencies emitted from one woman's vagina interfere with those of another, causing an unappealing positive feedback (aka vagitation) that attenuates the mood of all women contributing to the dissonance. The intensity of vaginal dissonance is directly proportional to 1) the average distance between each vagina, and 2) the average amplitude of emitted vaginal frequencies, which can vary dramatically between women. It is believed that most documented cases of PMS are actually mistaken episodes of vaginal dissonance.
Bob: "Dude, do not go in that room right now."
Bob: "Because my sister and her mother-in-law are both in there, and the vaginal dissonance is extreme."
Paul: "Damn...would you like to get a taco or something?"
Bob: "Would I!"