The lumbosacral joint is a joint of the body, between the last

lumbar vertebra The lumbar vertebrae are, in human anatomy, the five vertebrae between the rib cage and the pelvis. They are the largest segments of the vertebral column and are characterized by the absence of the foramen transversarium within the transverse ...

and the first sacral segment of the

vertebral column The vertebral column, also known as the backbone or spine, is part of the axial skeleton. The vertebral column is the defining characteristic of a vertebrate in which the notochord (a flexible rod of uniform composition) found in all chordate ...

. In some ways, calling it a "joint" (singular) is a misnomer, since the lumbosacral junction includes a disc between the lower lumbar vertebral body and the uppermost sacral vertebral body, as well as two lumbosacral facet joints (right and left zygapophysial joints).

References

Bones of the vertebral column Sacrum {{musculoskeletal-stub