Pelvis

[Sarah]

The pelvic arch, or the pelvic arch together with the sacrum.See Pelvic arch, under Pelvic, and Sacrum.

[Sarah]

That's your butt bone.

[Couch Potatoe]

the bones that form a bowl-shaped structure in the area below the waist at the top of the legs, and to which the leg bones and spine are joined

[Hina Muqadas]

The large bony structure near the base of the spine to which the hind limbs or legs are attached in humans and many other vertebrates.