1. Thinning of the bones with reduction in bone mass due to depletion of calcium and bone protein.
2. A decreased mass per unit volume of normally mineralized bone, leading to fractures after minimal trauma.

In other words, the bones are more porous than solid; similar to a natural sponge with a lot of holes.

Osteoporosis makes it much easier for a person to have bone fractures, which are often slow to heal and heal poorly or it is also described as an insidious disease that silently robs the skeleton of its mineral stores.

The term designates a condition wherein bone density measures two or more standard deviations less than normal.

The most common sites for such bone loss and resultant fractures are the thoracic and lumbar vertebrae, the ribs, the upper part of the femur, and the lower part of the radius. It is more common in older adults, particularly post-menopausal women; in patients on steroids; and in those who take steroidal drugs.

Unchecked osteoporosis can lead to changes in posture, physical abnormality (particularly the form of hunched back known colloquially as "dowager's hump"), and decreased mobility.

This bone condition can be detected by using tests that measure bone density. Treatment of osteoporosis includes ensuring that the diet contains adequate calcium and other minerals needed to promote new bone growth; as well as, exercises that build muscles, including walking, swimming, and (regardless of age) exercises that are available in "fitness studios".

Localized osteoporosis of the skull associated with Paget's disease (a disease of bone occurring in the middle aged and elderly; excessive bone destruction sometimes leading to bone pain and fractures and skeletal deformities).

Osteoporosis or a decrease in bone mass that occurs with sedentary people or patients resulting from a lack of normal functional stress on the bones.

It may occur during a prolonged period of bed rest or as the result of being exposed to periods of weightlessness; for example, astronauts in outer space.

Loss of bone tissue following trauma (physical damage to the body); especially, when there is damage to a nerve supplying the injured area.

The condition may also be caused by disuse secondary to pain.