There are several words that can be used as synonyms for the term "loss of physical fitness." Some of the most commonly used words include deterioration, decline, weakening, debilitation, and atrophy. These words all refer to a decline in physical ability or strength, whether it's related to age, injury, or illness. Other words that can be used to describe this condition include decay, enfeeblement, impairment, and languishment. Regardless of the specific synonym used, all of these terms indicate a decrease in physical ability that can often be addressed with exercise, physical therapy, or other forms of rehabilitation.