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Osteoporosis is primarily characterized by a decrease in bone mass, which leads to a reduction in bone density and structural integrity. This condition makes bones more fragile and susceptible to fractures, even with minor falls or injuries. The loss of bone mass occurs when the rate of bone resorption exceeds the rate of bone formation, resulting in weaker bones that can no longer support the body as effectively. This characteristic distinctly separates osteoporosis from conditions that might involve increased bone density or strength, as well as from those that focus on joint flexibility. Therefore, decreased bone mass is a fundamental aspect of osteoporosis, making it the correct answer.