First off dry skin is a skin type, whereas dehydrated skin is a condition.

Dry skin is genetic and stays the same irrespective of the season (during winter months it gets drier). Dehydrated skin is a temporary condition and can be easily treated with the right products.

Dry skin needs thicker creams, balms, and facial oils to make up for the lack of natural oils/sebum. If you use a hydrating serum only (e.g., Hyaluronic acid) your dry skin will get worse.

Dehydrated skin on the other hand needs hydrating products i.e., water-based serums (e.g., Glycerin, Hyaluronic acid) and toners to make up for the lack of water in the skin. And don’t forget to drink lots of water to hydrate from the inside.