Tutoring chemistry, definitions are always of interest. The tutor brings up the term anhydrous
anhydrous (adj): containing no water.
As I understand, the particles of an anhydrous substance are not associated with water, but likely would be if not for human intervention. Therefore, when something is described as anhydrous, it was likely put through a dehydrating (drying) process to become that way.
Source:Oracle Tutoring by Jack and Diane, Campbell River, BC.