Which condition is characterized by the weakening of bones due to loss of bone density?

The condition characterized by the weakening of bones due to a significant loss of bone density is osteoporosis. This condition occurs when the creation of new bone doesn't keep up with the removal of old bone, leading to a decrease in bone mass and increased fragility, making bones more susceptible to fractures.

Osteoporosis primarily affects older adults, particularly postmenopausal women, due to hormonal changes that affect bone density. While osteopenia represents a lower than normal bone density and often precedes osteoporosis, it does not yet reach the level of severity or risk associated with osteoporosis. Osteomalacia and rickets, on the other hand, are conditions related to softening of the bones due to vitamin D deficiency, affecting bone mineralization rather than bone density itself. Thus, osteoporosis is specifically linked to the deterioration of bone density, making it the correct answer.