- A small triangular bone at the base of the spinal column in humans and other apes, consisting of several fused rudimentary vertebrae.
- In <em>human anatomy</em>, the part of the spinal column consisting of the last four bones, the caudal vertebræ or tail-bones, which are stunted and usually ankylosed together. See cut under <internalXref urlencoded="skeleton">skeleton</internalXref>.
- In <em>comparative anatomy</em> and <em>zoology</em>, the caudal vertebræ, when few and small, or ankylosed together; the bony tail itself, when short, as in a bird.
- The end of the vertebral column beyond the sacrum in man and tailless monkeys. It is composed of several vertebræ more or less consolidated.
- The final fused <xref>vertebrae</xref> at the base of the <xref>spine</xref>.
- the end of the vertebral column in humans and tailless apes