 The category "c" is called a stack over the category "C" with a Grothendieck topology if it is a prestack over "C" and every descent datum is effective.A descent datum consists roughly of a covering of an object "V" of "C" by a family "Vi", elements "xi" in the fiber over "Vi", and morphisms "fji" between the restrictions of "xi" and "xj" to "Vij"="Vi"×"V""Vj" satisfying the compatibility condition "fki" = "fkjfji".The descent datum is called effective if the elements "xi" are essentially the pullbacks of an element "x" with image "V".