The lumbar region.
"After sitting at my desk all day, I felt a sharp ache in my lower back where the lumbar spine is located."
of or relating to or near the part of the back between the ribs and the hipbones
"lumbar vertebrae"
Related to the lower back or loin, specifically the five vertebrae between the rib cage and the pelvis.
"The doctor told me that my new sciatica pain is caused by a herniated disc in my lumbar region."