What disease is characterized by a reduction in bone mineral density of at least 2.5 standard deviations below the average for a given age range?

The condition characterized by a reduction in bone mineral density of at least 2.5 standard deviations below the average for a given age range is osteoporosis. This disease is primarily defined by its impact on bone strength and increased risk of fractures. Osteoporosis occurs when the creation of new bone doesn't keep up with the removal of old bone, leading to a significant decrease in bone mass and density.

The measurement of bone mineral density (BMD) is crucial in diagnosing osteoporosis; a T-score of -2.5 or lower indicates osteoporosis. This contrasts with osteopenia, which indicates lower than normal bone density but is less severe than osteoporosis, typically falling between -1.0 and -2.5 standard deviations from the average.

Arthritis and rheumatoid arthritis are terms that relate to joint inflammation and pain rather than bone density directly, thus they are not applicable to the definition provided in the question.