The standard deviation from the mean of a young (30-year old) adult is called a t-score.
If the t-score is -1, it means that the bone density is 1 standard deviation below the mean.
It is generally considered a t-score between -1 and -2.5 low bone density.
Patients with t-scores below -2.5 (e.g. -3) is considered suffering from osteoporosis. Also, patients within this range AND suffered from one or more fractures is considered established osteoporosis.