A phenomenon wherein the menstrual cycles of women living in close quarters tend to synchronize.
So named for the author of the first article to discuss the phenomenon, Menstrual Synchrony and Suppression, by Martha McClintock, which appeared in Nature in 1971. It seems she was at a conference where the phenomenon of synchronized ovulation in mice was broached. Ms. McClintock commented that the same thing happened in human females as well, as evidenced in dormitory life. While at first wary of the assertion, the scientists challenged her, as an undergraduate, to address the issue scientifically. McClintock did just that when she pursued the topic for her senior thesis at Wellesley and then published it when she was working on her graduate degree at Harvard.
This was the first paper to suggest any operation of pheromones in humans. While it did not definitively prove that pheromones were acting causally, it did offer tantalizing evidence. Further research by McClintock and others (Russell et al., 1980; Stern and McClintock, 1988; etc) did prove that it was pheromones causing the synchrony. Other studies have also found influences of male pheromones on women's menstrual cycles as well as evidence of female pheromones effecting men's hair growth.