Atrophy is the medical term for the loss of mass from any discrete part of the anatomy. It is a normal consequence of many conditions (such as muscle atrophy in a person with paralysis), but can often be a symptom of an underlying condition (such as the atrophy of the testicles which occurs when a man abuses testosterone to encourage muscle growth.
Atrophy describes an actual decrease and should not be used to describe a situation where something is merely smaller than normal, such as hypogonadism.