Osteopenia describes a lower than normal bone density. It's defined by having bone density lower than 70% of thirty year old females. It's most common in women after menopause.
Unless the patient suffers a broken bone, osteopenia is usually not problematic and does not indicate an underlying pathology. As such, there's a great deal of controversy as to whether the condition requires any treatment at all. It's theorized osteopenia will be followed by osteoporosis, which is a serious condition that is difficult to treat, but the research on this issue does not show a clear connection.