sacrum

n. triangular bone located at the rear of the pelvis between the lumbar vertebrae and the coccyx bone (Anatomy)

The sacrum is a large, triangular bone at the base of the spine and at the upper and back part of the pelvic cavity, where it is inserted like a wedge between the two hip bones. Its upper part connects with the last lumbar vertebra, and bottom part with the coccyx (tailbone). It is curved upon itself and placed obliquely (that is, tilted forward). It is kyphotic - that is, concave facing forwards. The base projects forward as the sacral promontory internally, and articulates with the last lumbar vertebra to form the prominent sacrovertebral angle. The central part is curved outward towards the posterior, allowing greater room for the pelvic cavity.
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Sacrum /sa·crum/ [L.] the triangular bone just below the lumbar vertebrae, formed usually by five fused vertebrae and lodged dorsally between the [more]Sacrum - Community and Resources

Noun
1. 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
(hypernym) bone, os
(part-holonym) pelvis, pelvic girdle, pelvic arch, hip
(part-meronym) sacral vertebra

n. sacrum, triangular bone located at the rear of the pelvis between the lumbar vertebrae and the coccyx bone (Anatomy)