Bone loss refers to a decrease in bone mass or density, which can lead to weakened and brittle bones. This condition is also known as osteoporosis, and it occurs when the body doesn't make enough new bone, or when too much old bone is removed. Bone loss can be caused by various factors such as aging, hormonal changes, lack of exercise, poor nutrition, certain medications, and chronic diseases like rheumatoid arthritis. If left untreated, bone loss can result in fractures, disability, and a higher risk of premature death.