Coccyx

The coccyx (/ˈkɒksɪks/ KOK-siks; plural: coccyges or coccyxes), commonly referred to as the tailbone, is the final segment of the vertebral column in humans and apes, and certain other mammals such as horses. In animals with bony tails, it is known as tailhead or dock, in bird anatomy as tailfan. It comprises three to five separate or fused coccygeal vertebrae below the sacrum, attached to the sacrum by a fibrocartilaginous joint, the sacrococcygeal symphysis, which permits limited movement between the sacrum and the coccyx.