Low bone density (sometimes called osteopenia) refers to bone density that is lower than the normal peak density but not low enough to be classified as osteoporosis. Bone density is a measurement of how dense and strong the bones are. If your bone density is low compared to normal peak density, you are said to have low bone density. Having low bone density means there is a greater risk that, as time passes, you may develop bone density that is very low compared to normal, known as osteoporosis.