Lumbarization is a term that refers to an anatomic anomaly in the human spine

Vertebral column

In human anatomy, the vertebral column is a column usually consisting of 24 articulating vertebrae, and 9 fused vertebrae in the sacrum and the coccyx. It is situated in the dorsal aspect of the torso, separated by intervertebral discs...

. It is defined by the nonfusion of the first and second segments of the sacrum

Sacrum

In vertebrate 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...

. The lumbar spine subsequently appears to have six vertebrae or segments, not five. Conversely the sacrum appears to have only four segments instead of its designated five segments.