What is the general term for any disease process that results in a reduction in the mass of bone per unit of volume?

The term for any disease process that results in a reduction in bone mass per unit of volume is osteoporosis. This condition is characterized by a decrease in bone density, making bones weaker and more susceptible to fractures. Osteoporosis occurs when the creation of new bone doesn't keep up with the removal of old bone, leading to a net loss of bone mass. It is particularly common in postmenopausal women due to hormonal changes, but it can affect anyone.

Osteomalacia refers to the softening of bones due to a deficiency of vitamin D, leading to improper mineralization. Osteitis is an inflammation of bone that can arise from infection, trauma, or other inflammatory processes; it does not specifically refer to a reduction in bone mass. Osteosarcoma is a type of bone cancer that results in the formation of new, abnormal bone tissue rather than a reduction in bone mass per unit of volume. Therefore, osteoporosis is the most accurate term for the condition described in the question.