Osteoporosis is a bone disease in which the amount of bone is decreased and the structural integrity of trabecular bone is impaired. Cortical bone becomes more porous and thinner. This makes the bone weaker and more likely to fracture.
In 1994, a committee of the World Health Organization defined osteoporosis based on bone density. Using standardized bone density measurements of the total hip, "normal" bone is greater than 833 mg/cm2. "Osteopenia" is between 833 and 648mg/cm2. Osteoporosis is lower than 648mg/cm2, and "Severe (established) osteoporosis" is when there has been a fragility fracture. Details are explained in the bone density section.
Now it is recognized that many factors lead to fractures, not just bone density. Age, heredity, body weight, diseases, lifestyle, frailty, and amount of trauma all play important roles. The risk of a fracture due to osteoporosis can be estimated using these factors in addition to the bone density.