One form of shock is hypovolemic shock in which the circulating blood volume is low. Dehydration may cause shock but is hardly synonymous with shock. Other forms of shock may have normal blood volume but the volume is maldistributed, e.g. anaphylactic shock. In some cases of shock the blood volume may in fact be increased but the heart may be unable to circulate the blood and oxygen to the tissues, e.g. cardiogenic shock. They just aren't the same thing at all.
Dehydration is the loss of total body water.
Hypovolemia is a low circulating blood volume. Total body water amounts to about 0.6 liters per Kg body weight. For the prototypical 70kg person that means a total body water of 42 l. Most of the time when we lose fluid we lose fluid from the intravascular space. Sweat glands for example take fluid from the blood and put it on the skin.