Dehydration deals with surface water loss. Truly dry skin reflects a lack of oil.
Many people think they have dry skin, because it feels dry, when really they are dehydrated.
This distinction becomes very important when buying skin care products. Truly dry skin is lack of oil. Dehydrated skin is lack of water. You can be oily and dehydrated, but you cannot be both oily and dry. Your oil glands are either producing enough, too much, or not enough oil.