Lordosis is an increased inward curving of the lumbar spine (just above the buttocks). The spine has three types of curves: lordotic, kyphotic (the outward curve of the thoracic spine at rib-level), and scoliotic (sideways curving).
A small degree of both kyphotic and lordotic curvature is normal. Too much lordotic curving is called swayback (lordosis). Too much kyphotic curving causes round shoulders or hunched shoulders (Scheuermann's disease).
Scoliotic curving is always abnormal. In the natural state of a healthy spine, the cervical spine has a lordotic curvature, the thoracic spine has a kyphotic curvature, and the lumbar spine has a lordotic curvature. Loss of the lumbar spine curvature is known as a “flat back”.