1.
the strong basin-shaped ring of bone near the bottom of the spine formed by the hip bones on the front and sides, and the triangular sacrum on the back
2.
any basin- or cup-shaped anatomical cavity, e.g. the region of the kidney into which urine is discharged before its passage into the ureter
3.
the large circular bones that support the lower part of your back and are connected to the bones of your legs