The lumbar spine is the lower back that begins below the last thoracic vertebra (T12) and ends at the top of the sacral spine, or sacrum (S1). Most people have 5 lumbar levels (L1-L5), although it is not unusual to have 6. Each lumbar spinal level is numbered from top to bottom—L1 through L5, or L6.
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Vertebrae – The 33 bones that make up the spine, individually referred to as a vertebra. They are divided into the cervical spine (neck), the thoracic spine (upper back or rib cage), the lumbar spine (lower back) and the sacral spine (pelvis or base of the spine).
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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 process (since it is only found in the cervical region) and by the absence of facets on the sides of the body (as found only in the thoracic region). They are designated L1 to L5, starting at the top. The lumbar vertebrae help support the weight of the body, and permit movement.
https://en.wikipedia.org/wiki/Lumbar_vertebrae#:~:text=Lumba...
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The sacrum ; plural: sacra or sacrums[1]), in human anatomy, is a large, triangular bone at the base of the spine that forms by the fusing of sacral vertebrae S1–S5 between 18 and 30 years of age.[2]

https://en.wikipedia.org/wiki/Sacrum