What term describes a condition of reduced activity or being out of use?

The term "desuetude" specifically denotes a state of disuse or inactivity. It originates from the Latin word "desuetus," meaning "unaccustomed." When something is in desuetude, it implies that it has fallen into neglect or has not been used for a significant period. This term captures the essence of being inactive or rendered obsolete.

In contrast, desultory refers to a lack of a definite plan or purpose, while dilatory pertains to being slow to act or causing delays. Diaphanous describes something that is light, delicate, and translucent, often used in the context of fabrics or materials. Therefore, desuetude is the most appropriate and accurate choice for the condition of being out of use.