If you have a "Dry" skin type, it is lacking in oil. Dehydrated skin on the other hand is characterized by a lack of moisture in the Stratum corneum (the outermost layer of the epidermis).

The Stratum corneum cells normally contain about 10% water. Anything below that is classified as a "Dehydrated" skin condition.

Because Dehydration is really a lack of water in the skin, it can even be experienced by those whose skin type is "Oily".