What term describes a decrease in the size of a structure that was once normal in dimension?

The correct term that describes a decrease in the size of a structure that was once normal in dimension is atrophy. Atrophy refers to the wasting away or reduction in size of a tissue or organ, which can occur due to various factors such as disuse, lack of nutrients, or disease. In a physiological context, when muscles or other tissues are not used regularly or sufficiently, they can shrink in size and functionality.

Hypertrophy, in contrast, refers to an increase in the size of an organ or tissue, usually in response to increased workload or stress. Dystrophy implies a disorder in the development of tissue, which can lead to abnormal growth or function, rather than simply a decrease in size. Stenosis refers to the constriction or narrowing of a passage or opening, not a decrease in size of a structure that was previously normal. Therefore, atrophy is the most appropriate term for the situation described in the question.