1. The column of bone known as the vertebral column, which surrounds and protects the spinal cord.

The spine can be categorized according to the level of the body; for example, the cervical spine (neck), the thoracic spine (upper and middle back), and the lumbar spine (lower back).

2. Any short prominence of bone.

The spines of the vertebrae protrude at the bottom of the back of the neck and in the middle of the back. These spines protect the spinal cord from injury from behind.