Definition of the term "Zav"
A man may see a white discharge from his male member, more white than semen, which is called "zivah," or "flow." When he sees it, he has ritual impurity similar to one who had a seminal emission.
If he sees the discharge twice in one day, or on two consecutive days, he is called a "zav." The ritual impurity of a "zav" is stronger than that of the one who had a seminal emission, and in additional he must wait for seven days before becoming pure. A regular mikveh is not enough for him, and he must immerse himself in running spring-water. If he sees three discharges, he observes all of the rules above, and must additionally bring a sacrifice on the eight day.