**Core Concept**
Osteoporosis is a condition characterized by a decrease in bone mass and density, leading to an increased risk of fractures. In postmenopausal women, the diagnosis of osteoporosis is often made using dual-energy X-ray absorptiometry (DXA) scans, which measure bone mineral density (BMD). The T-score is a key metric used to diagnose osteoporosis.

**Why the Correct Answer is Right**
A T-score of -2.5 or lower is used to define osteoporosis in postmenopausal women. This cutoff is based on the World Health Organization (WHO) definition of osteoporosis, which takes into account the standard deviation of BMD in young, healthy women. A T-score of -2.5 or lower indicates that the patient's BMD is 2.5 standard deviations below the mean for young, healthy women, putting them at increased risk for fractures.

**Why Each Wrong Option is Incorrect**
**Option A:** A T-score above -1 is considered normal, so this option is incorrect.
**Option B:** A T-score between -1 and -2.5 indicates osteopenia, a condition of low bone mass that is not severe enough to be classified as osteoporosis.
**Option D:** This option is not a valid definition of osteoporosis.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that osteoporosis is a significant risk factor for fractures, particularly in postmenopausal women. Early diagnosis and treatment can help prevent fractures and improve quality of life.

**Correct Answer:** C. -2.5 or lower