The absorption of water by an anhydrous compound is called hydration.

Here's a breakdown:

* Anhydrous: Means "without water." This refers to a compound that does not contain any water molecules.

* Hydration: The process of adding water molecules to a substance. In the case of anhydrous compounds, it's the process of absorbing water molecules.

For example, when copper sulfate (CuSO₄) is anhydrous, it's a white powder. Upon hydration, it absorbs water molecules and forms copper sulfate pentahydrate (CuSO₄•5H₂O), which is a blue crystal.