DEXA scores are reported as "T-scores" and "Z-scores."
The T-score is a comparison of a person's bone density with that of a healthy 30-year-old of the same sex.
The Z-score is a comparison of a person's bone density with that of an average person of the same age and sex.
Lower scores (more negative) mean lower bone density:
A T-score of -2.5 or lower qualifies as osteoporosis.
A T-score of -1.0 to -2.5 signifies osteopenia, meaning below-normal bone density without full osteoporosis.
~WebMD website definition~