sacrum

nouns

wedge-shaped bone consisting of five fused vertebrae forming the posterior part of the pelvis; its base connects with the lowest lumbar vertebra and its tip with the coccyx
sacrum
is derived from
sacral
is a part of
the structure of the vertebrate skeleton supporting the lower limbs in humans and the hind limbs or corresponding parts in other vertebrates
hip, pelvic arch, pelvic girdle, pelvis
is a type of (narrower)
rigid connective tissue that makes up the skeleton of vertebrates
bone, os