S1-S5 - Sacral (sa´krul) vertebrae:

The first sacral vertebra through the fifth sacral vertebra. There are 5 sacral vertebral bones. They are represented by the symbols S1 through S5. The sacral vertebrae are situated in the spinal column just below the lumbar vertebrae and right above the coccyx, which is the lowest segment of the vertebral column. The sacral vertebrae are normally found fused together to form the sacrum.

The sacrum is the triangular bone at the base of the spine formed by usually five fused vertebrae wedged dorsally between the two hip bones.